Application and investigation of thrombopoiesis-stimulating agents in the treatment of thrombocytopenia

Abstract

Platelets, derived from a certain subpopulation of megakaryocytes, are closely related to hemostasis, coagulation, metastasis, inflammation, and cancer progression. Thrombopoiesis is a dynamic process regulated by various signaling pathways in which thrombopoietin (THPO)–MPL is dominant. Thrombopoiesis-stimulating agents could promote platelet production, showing therapeutic effects in different kinds of thrombocytopenia. Some thrombopoiesis-stimulating agents are currently used in clinical practices to treat thrombocytopenia. The others are not in clinical investigations to deal with thrombocytopenia but have potential in thrombopoiesis. Their potential values in thrombocytopenia treatment should be highly regarded. Novel drug screening models and drug repurposing research have found many new agents and yielded promising outcomes in preclinical or clinical studies. This review will briefly introduce thrombopoiesis-stimulating agents currently or potentially valuable in thrombocytopenia treatment and summarize the possible mechanisms and therapeutic effects, which may enrich the pharmacological armamentarium for the medical treatment of thrombocytopenia.

Keywords

drugs thrombocytopenia thrombopoiesis thrombopoiesis-stimulating agents

Introduction

Thrombopoiesis, the development of platelets, occurs primarily within bone marrow (BM) and lung.¹ Platelets may be derived from a certain subset of megakaryocytes (MKs) expressing genes associated with proplatelet production and platelet function.² Once released by high-ploidy MKs, platelets function as regulators of hemostasis, thrombosis, metastasis, inflammation, and cancer progression.³ Several factors regulate thrombopoiesis. GATA binding protein 1 (GATA1), friend leukemia virus integration 1 (Fli-1), runt-related transcription factor 1 (RUNX1), and Rap1 GTPases activator C3G play a role in regulating megakaryopoiesis.⁴ C-myb (MYB), which adjusts and balances the development of MKs and GATA-1, and Tribbles Pseudokinase 3 gene (TRIB3) are negative modulators of megakaryopoiesis.⁵ Thrombopoietin (THPO) is the primary regulating cytokine in thrombopoiesis.⁶ Since THPO binds to the myeloproliferative leukemia(MPL) receptor, several downstream signaling pathways consisting of mitogen-activated protein kinase (MAPK)/ extracellular signal-regulated kinases (ERK), PI3K-Akt, and Janus kinase (JAK)/STAT which participate in MK proliferation, differentiation, maturation, and functional platelet division are activated.⁷

Thrombocytopenia, characterized by platelet counts less than 150 × 10³/μl, is related to decreased platelet production, increased destruction, splenic sequestration, dilution, or clumping.⁸ Severe thrombocytopenia may lead to low platelet counts and high bleeding risk, and cause life-threatening bleeding events. Thrombopoiesis-stimulating agents are vital in the drug treatment of thrombocytopenia. In this review, thrombopoiesis-stimulating agents are classified according to their research progress in treating thrombocytopenia. Some drugs are already in clinical trials for the treatment of thrombocytopenia, such as THPO, thrombopoietin receptor agonists (THPO-RAs), all-trans retinoic acid (ATRA), and interleukin-11 (IL-11)-based agents, which have manifested reliable therapeutic effects in different kinds of thrombocytopenia, including immune thrombocytopenia (ITP), chemotherapy-induced thrombocytopenia (CIT), inherited thrombocytopenia (IT), post-hematopoietic stem cell transplantation (HSCT) thrombocytopenia, presurgical thrombocytopenia, and thrombocytopenia related to chronic liver diseases (CLD), aplastic anemia (AA), radiation, and sepsis (Table 1). Another group of agents includes some drugs and natural products which are found to promote thrombopoiesis but are still not in clinical investigations to deal with thrombocytopenia (Table 2). Their various mechanisms of action in promoting thrombopoiesis (Figures 1 and 2) and other indications apart from thrombocytopenia will be briefly introduced to show the multifunction of these agents (Tables 3 and 4). However, their potential in thrombocytopenia treatment should be highly valued.

Table 1.

Thrombopoiesis-stimulating agents for clinical treatment of thrombocytopenia.

Class of agents	Name	Suggested mechanism(s) in stimulating thrombopoiesis	Suggested effective type(s) of thrombocytopenia
THPO	PEG-recombinant human MK growth and development factor (PEG-rHuMGDF)	Activates the THPO–MPL signaling	CIT
	Recombinant human THPO (rHuTHPO)	Activates the THPO–MPL signaling	CIT
THPO-RAs	Romiplostim	Activates the THPO–MPL signaling	ITP, CIT, IT, post-HSCT thrombocytopenia, thrombocytopenia related to CLD, AA, acute radiation syndrome, presurgical use
	Eltrombopag	Activates the THPO–MPL signaling	ITP, IT, post-HSCT thrombocytopenia, AA
	Avatrombopag	Activates the THPO–MPL signaling	ITP, thrombocytopenia related to CLD, presurgical use
	Lusutrombopag	Activates the THPO–MPL signaling	Thrombocytopenia related to CLD, presurgical use
	Hetrombopag	Activates the THPO–MPL signaling	ITP
IL-11-based agents	Recombinant human IL-11 (RhIL-11)	Activates the gp130 signaling, JAK/TYK, and MAPK signaling	ITP, CIT, thrombocytopenia related to sepsis
	Hyper-IL-11 (H11)	Activates the gp130 signaling, JAK/TYK, and MAPK signaling	In development
	PEGylated IL-11	Activates the gp130 signaling, JAK/TYK, and MAPK signaling	In development
Retinoids	ATRA	Respond to thrombin receptor-activating peptide, may be dependent with THPO	ITP

AA, aplastic anemia; CIT, chemotherapy-induced thrombocytopenia; CLD, chronic liver diseases; ITP, immune thrombocytopenia; THPO, Thrombopoietin.

Table 2.

Thrombopoiesis-stimulating agents with potential value of clinical use.

Class of agents	Name	Suggested mechanism(s) in stimulating thrombopoiesis
Catecholamine	Dopamine (DA)	Activates the p38 MAPK, JNK/SAPK, and caspase-3 pathways
	Norepinephrine (NE)	Activates the α2-adrenoceptor-mediated ERK1/2 signaling, then activates the RhoA GTPase signaling
	Epinephrine (EPI)	Activates the α2-adrenoceptor-mediated ERK1/2 signaling, then activates the RhoA GTPase signaling
Hormones	Estrogen	Activates the ERβ-dependent pathway, activates GATA-1 and STAT1
	Melatonin	Activates the ERK1/2 and Akt signaling
	Recombinant human growth hormone (rhGH)	Activates the ERK1/2 and Akt signaling
Benzodiazepines	Clozapine	Activates the IL-6-induced gp130 signaling
Yes-associated protein (YAP) inhibitors	Dobutamine	Inhibits YAP in the Hippo pathway, upregulation of Fli-1
	Verteporfin	Inhibits YAP in the Hippo pathway, upregulation of Fli-1
Natural agents	Ingenol	Activates the PI3K-Akt signaling
	3,3′-di-O-methylellagic acid 4′-glucoside (DMAG)	Activates the PI3K-Akt signaling

Figure 1.

Mechanisms of thrombopoiesis-stimulating agents for clinical treatment of thrombocytopenia.

Figure 2.

Possible mechanisms of thrombopoiesis-stimulating agents with potential value of clinical use.

Table 3.

Other indications and mechanisms of action of some thrombopoiesis-stimulating agents for clinical treatment of thrombocytopenia.

Name	Indication(s)	Mechanism(s)
RhIL-11	Tissue protector in anticancer therapy (enhance the recovery of BM, oral epithelium, and intestinal crypt cells after cytotoxic insult by anticancer drugs or ionizing radiation)	Activates the gp130 signaling, JAK/TYK, and MAPK signaling;Upregulates anti-apoptotic gene BCL-2, downregulates apoptotic gene BAX;Induces IEC-6 cell cycle arrest
ATRA	Psoriasis, acne, and ichthyosis	Stimulates FOXO1 gene expression
	Bronchial asthma, inflammatory bowel disease (IBD), periodontitis	Regulates the cellular function of Th17/Treg
	Acute promyelocytic leukemia (APL)	Activates the transcription factor FOXO3A in APL cells

Table 4.

Other indications and mechanisms of action of some thrombopoiesis-stimulating agents with potential value of clinical use.

Name	Indication(s)	Mechanism(s)
DA	Cardiovascular diseases (hypertrophy, myocardial ischemia, hypertension, and arrhythmia), acute renal failure	D1R (activates cAMP/PKA signaling, Gs/PLC/PKC signaling);D2R (activates Gi-Go/AC, suppressing cAMP)
NE	Neurogenic shock, hypotension after pheochromocytoma resection or drug intoxication, hemostatic	Activates α-R
EPI	Cardiac arrest, anaphylactic disease (anaphylactic shock, Bronchial asthma, angioneuroedema), glaucoma, hemostatic	Activates α-R/β-R
Estrogen	Perimenopausal syndrome; osteoporosis, ovarian dysfunction, breast cancer (post-menopause), prostate cancer, functional uterine bleeding, acne	Activates ER
Melatonin	Dyssomnia	Activates MT1 and MT2
	Neurological diseases (Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, amyotrophic lateral sclerosis, Huntington’s disease, epilepsy, headache, etc.)	Free-radical scavenger, antioxidant, anti-inflammation, immune suppression
	Different types of cancer (reduction in tumor growth and metastases, reduction in the side effects associated with chemotherapy and radiotherapy, decreasing drug resistance in cancer therapy, and augmentation of the therapeutic effects of conventional anticancer therapies)	Antioxidant, antiproliferation (downregulates NF-κB/iNOS, PI3K, and MAPK signaling pathways, inhibits SPHK1)Promoting cell apoptosis (activates BAX/BAK, Apaf-1, caspases, and p53)
	Cardiovascular risk factors and diseases (diabetes, hypertension, hyperlipidemia, obesity, myocardial ischemia–reperfusion injury, pulmonary hypertension, and atherosclerosis)	Antioxidant, anti-inflammation, regulates metabolism
	Bone-related diseases (fractures, osteoporosis, and osteoarthritis)	Antioxidant, anti-inflammation, promoting wound healing and tissue regeneration, regulation of bone mass accumulation and loss
rhGH	Growth hormone deficiency (GHD), short stature (SS)	Activates GHR
Clozapine	Antipsychotics	Activates DR4 and serotonin-DA receptor
Dobutamine	Heart failure, septic shock	Activates β1-R
Verteporfin	Age-related macular degeneration (AMD), central serous chorioretinopathy (CSC), ocular histoplasmosis syndrome, and pathologic myopia	Photodynamic therapy
	Different types of cancer (retinoblastoma, gliomas, breast cancer, etc.)	Inhibits YAP in the Hippo pathway
Ingenol	Warts, skin keratoses, and skin cancer	Activates PKC-α/δ
DMAG	Hepatoprotective	Antioxidant

Thrombopoiesis agents for clinical treatment of thrombocytopenia

THPO and THPO-RAs

THPO

THPO plays an essential part in thrombopoiesis. Through binding to the c-MPL receptor, it can make a difference in the HSC differentiation and survival, MKs differentiation, and maturation.⁹ Given the importance of THPO in promoting thrombopoiesis, PEG-recombinant human MK growth and development factor (PEG-rHuMGDF) and recombinant human THPO (rHuTHPO) are developed for thrombocytopenia treatment. Several clinical trials proved the effect of rHuTHPO in preventing and treating CIT.^10,11 PEG-rHuMGDF has comparatively longer half-life, which is mainly used for CIT in patients with cancer or hematological malignancies as an alternative for rHuTHPO.^12,13 However, the usage of these two recombinant THPO is both limited due to the potential risk of antibody formation to endogenous THPO.¹⁴

THPO-RAs

THPO-RAs are drugs that bind to THPO receptors, promoting megakaryopoiesis and platelet production.¹⁵ The increasing use of THPO-RAs, such as romiplostim, eltrombopag, and avatrombopag, shows promising effects in ITP, AA, CIT, thrombocytopenia in CLD, IT, post-HSCT thrombocytopenia, and perioperative use for reducing platelet infusion.^16,17 Novel THPO-RAs with better effect and safety, such as lusutrombopag and hetrombopag, are under clinical investigation for thrombocytopenia treatment.

Romiplostim is a peptide which binds to the extra-cytoplasmic domain of the THPO receptor, activating various signaling pathways in mature MKs, such as JAK2/STAT5, PI3K-Akt, ERK, and STAT3.¹⁵ Since romiplostim does not have amino acid sequence homology to endogenous THPO, it lowers the risk of THPO antibody formation and cross-reactivity. An analysis of data from five clinical trials in pediatric ITP patients and postmarketing registry shows that romiplostim’s immunogenicity occurs infrequently and has no significant association with the deficiency of platelet response or other adverse events.¹⁸ Another analysis from nine clinical studies, including 311 patients with ITP for at most 1 year (277 in romiplostim group) and 726 for more than 1 year (634 in romiplostim group), has proved that romiplostim can be effectively applied in ITP treatment. Platelet counts rose and remained stably elevated in most patients in romiplostim group. What is more, platelet response, which was indicated by platelet counts of at least 50 × 10⁹/liter, occurred in 86% of patients with ITP ⩽ 12 months and 87% of patients with ITP > 12 months, reached a significant increase compared with both placebo group and standard of care group.¹⁹ However, the rate of bleeding events was lower in romiplostim group than that of placebo group and standard of care group. The occurrence rates of thrombotic events for all groups were at similar level, independent of ITP duration.¹⁹ Data from these clinical studies have proved the definite and safe effect of romiplostim in different phases of ITP treatment. Romiplostim also has potency in treating various types of thrombocytopenia, including CIT, MYH9-related IT, AA, acute radiation syndrome, presurgical thrombocytopenia, post-HSCT thrombocytopenia, and thrombocytopenia related to CLD.^20–22

Eltrombopag is a small molecule and functions within the transmembrane and juxtamembrane domains of the THPO receptor through associating with specific amino acids and metal ions, so that the THPO receptor is activated.²³ Unlike romiplostim that mainly acts in mature MK, eltrombopag usually acts in earlier MK and late MK, stimulates MK precursor and MK differentiation, activates JAK2/STAT5, PI3K-Akt, and ERK1/2 pathways by increasing phosphorylation, and promotes thrombopoiesis.^15,24 It is found that eltrombopag is able to maintain platelet counts and make critical adverse events in ITP treatment less frequent. As a real-world study in Japan has demonstrated, when eltrombopag treatment, – no matter monotherapy or combination with other medicine, such as corticosteroids – was applied, the risk of bleeding-related episodes was about 30% lower than that of corticosteroid monotherapy in patients with ITP.²⁵ Apart from ITP, a phase II clinical trial investigated that eltrombopag was capable of increasing platelet counts while reducing bleeding events in the treatment of IT. Among 24 patients with different forms of IT with baseline platelet counts of 40.4 × 10⁹/liter, including ITGB3-related thrombocytopenia, monoallelic Bernard–Soulier syndrome, X-linked thrombocytopenia/Wiskott–Aldrich syndrome, ANKRD26-related thrombocytopenia, or MYH9-related disease, 11 patients realized a major response of platelet counts over 100 × 10⁹/liter, 10 patients achieved a minor response with platelet counts being no less than two times the baseline value.²⁶ Eltrombopag is also valid and safe for raising the platelet counts for post-HSCT thrombocytopenia in adult or pediatric patients. There may be a positive correlation between the number of MKs in the BM before treatment and therapeutic effect.^27,28 Combining eltrombopag and anti-thymocyte globulin (ATG) resulted in better safety and effect than using ATG alone to treat severe AA.²⁹ Vitro colony-forming units of MKs assay suggested that myelodysplastic syndrome (MDS) patients may benefit from eltrombopag combined with azacitidine.³⁰ These research broaden the vision of its potential in thrombocytopenia and other hematopoietic diseases.

Avatrombopag is a small-molecule THPO-RA. The different binding site of avatrombopag from endogenic THPO could avoid competitive inhibition and contribute to the additivity result of promoting thrombopoiesis.³¹ In contrast to eltrombopag, avatrombopag is incapable of chelating iron or cations.³² A network meta-analysis for patients with chronic ITP revealed that avatrombopag is associated with statistically significant improvements in either platelet response (platelet counts ⩾ 50,000/µl) or complete response (platelet counts ⩾ 100,000/µl) in different stage of treatment, including day 8, day 28, and month 6, compared with the placebo group.³³ In addition, 64.0% of patients of avatrombopag group were observed to have durable clinically relevant response which means platelet counts of ⩾ 30,000/µl will last for 6 weeks during the final 8 weeks of the core study, while the number of placebo group is 0%.³³ It is found that concomitant ITP medication, such as corticosteroids, was less applied in patients with avatrombopag treatment.³³ Avatrombopag is also effective for treating adult patients who suffer from CLD since it is capable of increasing platelet levels in thrombocytopenia, significantly reducing platelet transfusions or rescue procedures.³⁴ A lower occurrence of bleeding events has been shown compared with romiplostim and eltrombopag in patients with chronic ITP, indicating avatrombopag’s better safety in clinical use.³⁴ Avatrombopag is also approved for using before surgeries, such as splenectomy, radiofrequency ablation of the spleen, and partial splenic artery embolization.³⁴

Lusutrombopag is a small-molecule THPO-RA on the human THPO receptor, which can act selectively and promote the proliferation and differentiation of BM cells into MKs, increasing the platelet levels in thrombocytopenia patients.³⁵ Lusutrombopag could be used to deal with mild-to-moderate thrombocytopenia in CLD patients. Its therapeutic effect was proved by a clinical trial of CLD patients with different severity of thrombocytopenia. The effect was the same for mild thrombocytopenia (platelet counts ⩾ 50,000/µl) and moderate thrombocytopenia (platelet counts < 50,000/µl).³⁶ However, as reported from a multicenter clinical study of 139 patients with thrombocytopenia caused by CLD, lusutrombopag has better effect on mild thrombocytopenia than on severe thrombocytopenia.³⁷ Lusutrombopag is safe and productive for increasing platelet counts, reducing bleeding events and decreasing the need for platelet transfusion of patients with or without hepatocellular carcinoma (HCC), which is a secondary disease for CLD, according to an analysis of data from two phase III clinical trials.³⁸ It is approved for presurgical use with indications similar to avatrombopag, as alternatives to platelet transfusion.³⁴

Hetrombopag is a novel, active, small-molecule THPO-RA. It promotes MKs proliferation and differentiation, and offers an additive thrombopoiesis-stimulating effect in combination with rHuTHPO.³⁹ A multicenter, randomized phase III trial of hetrombopag showed its efficacy and good tolerance in treating ITP patients who had no response or relapse after previous treatment with realizing a platelet response (58.9% in once-daily 2.5 mg hetrombopag group and 64.3% in once-daily 5 mg hetrombopag group) and bringing down the risk of bleeding and the requirement of rescue therapy through 8 weeks’ treatment, compared with placebo.⁴⁰

IL-11-based agents

IL-11 is a cytokine which belongs to interleukin-6 (IL-6) family with hematopoietic proliferative properties. It is crucial for promoting MK colony formation and plays a stimulating effect on thrombopoiesis.⁴¹ Various IL-11-based agents, such as recombinant human IL-11 (RhIL-11), hyper-IL-11 (H11), and PEGylated IL-11, are currently or potentially used to treat thrombocytopenia.

The recombinant form of IL-11, RhIL-11, signals through the gp130 subunit of the receptor. Co-expression of the IL-11 receptor α-chain and gp130 results in high-affinity binding, initiating signal transduction of the JAK/TYK and the MAPK.⁴² Due to RhIL-11’s inducing MKs maturation and differentiation, and normalizing Th1/Th2 and T-bet/GATA-3 ratios close to healthy states,⁴³ it shows therapeutic effect in ITP, CIT, and thrombocytopenia in sepsis patients.^44–46 RhIL-11 also acts as a tissue protector in anticancer therapy because it activates the gp130 signaling, JAK/TYK, and MAPK signaling, downregulates apoptotic gene BAX, upregulates anti-apoptotic gene BCL-2, and induces IEC-6 cell cycle arrest,^47,48 although the main indication of RhIL-11 is treating thrombocytopenia.

As a fusion protein comprising IL-11, H11 targets the gp130 signal-transducing subunit.⁴⁹ Compared with rhIL-11, H11 shows more effectiveness in strengthening early progenitors’ expansion and directing them to MKs, inducing the maturation of MKs and the release of platelet-like particles (PLPs). Therefore, H11 may be more productive for stimulating the MK colony formation and PLPs production than RhIL-11, that may be beneficial for thrombocytopenia treatment and an ex vivo expansion of MKs.⁵⁰ Compared with IL-11, PEGylated IL-11 not only has a longer half-life but also displays its ability of inducing longer-lasting increases in hematopoietic cells.⁵¹ H11 and PEGylated IL-11 may have better potent effects on stimulating thrombopoiesis, but further clinical evidence of H11 and PEGylated IL-11 in thrombocytopenia treatment is needed compared with RhIL-11.

ATRA

ATRA is commonly used to treat skin diseases, inflammatory diseases, and acute promyelocytic leukemia (APL).⁵² It relieves psoriasis, acne, psoriasis and ichthyosis by stimulating FOX1 gene expression,^53,54 regulates the cellular function of Th17/Treg to treat bronchial asthma, inflammatory bowel disease (IBD), periodontitis,^55–57 and activates the transcription factor FOXO3A in APL cells.⁵⁸ Concentration of 10⁻¹² M ATRA could stimulate thrombopoiesis from hematopoietic progenitors in the presence of THPO or granulocyte-macrophage colony-stimulating factor (GM-CSF) plus IL-3.⁵⁹ ATRA could stimulate thrombopoiesis in MEG-01 cell by promoting MK differentiation and generation of functional PLPs. These functional PLPs could be activated by thrombin receptor-activating peptide 6 (TRAP-6) with increased externalization of P-selectin.⁶⁰ An increasing level of serum THPO was observed in APL patients with ATRA treatment, resulting in an increase in platelet counts.⁶¹ A phase II clinical trial of 66 patients who were newly diagnosed with primary ITP reported that the proportion of patients with a sustained response (maintaining platelet counts at the level of at least 30 × 10⁹/liter and at least twice higher than the baseline counts, without bleeding events and no rescue medication being required in 6 months) in the ATRA plus high-dose dexamethasone group (68%) was significantly higher than that of high-dose dexamethasone monotherapy group (41%), representing a novel therapeutic regimen for ITP.⁶² Compared to low-dose rituximab (LD-RTX) monotherapy, ATRA plus LD-RTX increased the overall and sustained response for corticosteroid-resistant or relapse ITP patients (80% versus 59% of overall response and 61% versus 41% of sustained response).⁶³ These clinical investigations demonstrated ATRA might stimulate thrombopoiesis through a THPO-dependent manner, which may be beneficial in relieving thrombocytopenia, and drug combination is necessary.

Thrombopoiesis agents with potential value of clinical use

Catecholamine

Catecholamines are a group of tyrosine-derived neurotransmitters, including dopamine (DA), norepinephrine (NE), and epinephrine (EPI). DA has therapeutic effects on cardiovascular diseases (hypertrophy, myocardial ischemia, hypertension, and arrhythmia) and acute renal failure.⁶⁴ NE and EPI are commonly used to deal with severe allergic reactions, septic shock, and cardiopulmonary resuscitation.^65–67 Animal experiment demonstrated that EPI and NE in response to sympathetic stimulation could promote platelet production by the activation of α2-adrenoceptor-mediated ERK1/2 signaling, which induces MKs adhesion and migration, and the ERK1/2 activation-mediated RhoA GTPase signaling could stimulate proplatelet formation (PPF), platelet release, and platelet activation.⁶⁸ However, the insufficient evidence of the effect of exogenous NE and EPI on platelet production requires further investigation to clarify the association between medicinal NE and EPI with thrombopoiesis. As the precursor of NE and EPI, DA cannot accelerate the early stages in thrombopoiesis proceedings but promote production of platelet from MKs in the final stages of thrombopoiesis, relating to oxidative stress-mediated signaling pathways which consist of p38 MAPK, JNK/SAPK, and caspase-3 pathways in human primary MKs. In addition, administration of DA in mice could elevate platelet counts.⁶⁹ These findings of catecholamines on stimulating thrombopoiesis provide new insights into drug treatment of thrombocytopenia.

Estrogen

Estrogen, one of the primary sex hormones, plays various effects in relieving perimenopausal syndrome, osteoporosis, ovarian dysfunction, functional uterine bleeding, breast cancer (post-menopause), prostate cancer, and acne.^70–72 Estrogen receptors (ER), including ERα and ERβ, which express on MKs, could mediate the promoting effect of MKs differentiation, PPF, and platelet production.^73,74 Estrogen-induced platelet production may be regulated by the ERβ-dependent pathway. Through detecting BM-derived MKs and Meg-01 cells of estrogen-treated mouse, it could be found that estrogen acts as a potential activator of GATA-1, then promotes the activation of STAT1 for MKs polyploidization, maturation, and platelet production. That is consistent with the result that platelet counts in estrogen-treated mice had a significant increase.⁷⁵ Evidence from clinical cases in postmenopausal women receiving estrogen replacement therapy showed an increase in MK numbers,⁷⁶ but the stable effect of estrogen on elevating platelet counts in human, especially thrombocytopenia patients still require further investigation.

Melatonin

Melatonin is a hormone synthesized and secreted mainly from the pineal gland.⁷⁷ It is mainly used as a drug to treat sleep disorders, such as insomnia and circadian rhythm disorders.⁷⁸ It could be potentially beneficial to the treatment of neurological diseases, cardiovascular diseases, bone diseases, and cancers.^79–82 Therapeutic potential of melatonin in thrombocytopenia is under development. Melatonin could promote thrombopoiesis by increasing HSCs and MKs expansion, PPF, and platelet formation.⁸³ From a molecular perspective, melatonin-induced thrombopoiesis might be mediated by ERK1/2 and Akt signaling.⁸³ Besides stimulating thrombopoiesis, melatonin could inhibit MKs apoptosis by activating Akt signaling through melatonin receptors.⁸⁴ Recent clinical evidence supported that compared with placebo, melatonin treatment could significantly enhance platelet counts (from 175.67 ± 92.84 to 191.10 ± 98.82) in patients who were affected by liver disease, reflecting its potential in thrombocytopenia treatment.⁸⁵ Further clinical evidence on patients with different kinds of thrombocytopenia is required, and melatonin is hopeful to become novel choice in the drug treatment of thrombocytopenia.

Recombinant human growth hormone

Recombinant human growth hormone (rhGH) is used for replacing treatment of growth hormone deficiency.⁸⁶ It also acts as the thrombopoiesis-stimulating agent which promotes various steps in platelet development, including the late stage of MK differentiation, PPF, and production of platelet.⁸⁷ Treatment of rhGH could activate STAT5. In an early phase, activation of ERK1/2 proceeds slowly but in a prolonged period in M07e megakaryoblastic cells and primary MKs treated with rhGH. However, activation of Akt was only observed in mature MKs but not in M07e megakaryoblastic cells, implying that the terminal differentiation of MKs induced by rhGH may be attributed to the Akt pathway.⁸⁷ Moreover, through significantly accelerating and promoting platelet production, rhGH was manifested to strengthen the effect of a tandem dimer of thrombopoietin mimetic peptide (dTMP) on thrombopoiesis in M07e cells and mice model of radiation-induced thrombocytopenia (RIT), denoting that rhGH and c-MPL may achieve thrombopoiesis complementarily and synergistically.⁸⁷ More clinical evidence is required to confirm the promoting effect of rhGH on relieving thrombocytopenia, especially in combination with THPO and THPO-RAs.

Clozapine

Clozapine is an antipsychotic drug effective for resistant schizophrenia.⁸⁸ However, its stimulating effects on the hematopoietic system, such as agranulocytosis, leukocytosis, and thrombocytosis, are recognized as significant side effects.^89,90 A follow-up study about clozapine’s stimulating effect on patients’ thrombopoiesis of a 1-year period demonstrated that since the beginning of clozapine treatment in the first week, platelet counts increased synchronically but transiently, and the early spike in platelet counts reflected raised IL-6 levels, which induced thrombopoiesis through the gp130 signaling and interaction with IL-3 and THPO.^91,92 This result suggests that IL-6 might be the crucial factor in clozapine-induced thrombocytosis. However, the direct evidence of clozapine’s interactions with IL-6 or other important factors such as THPO to promote thrombopoiesis is required for affirming the relevance of clozapine to the platelet development process.

Yes-associated protein inhibitors

Yes-associated protein (YAP) is an effector molecule in the Hippo pathway, which plays an essential role in MKs proliferation, differentiation, and production of platelet.⁹³ Dobutamine and verteporfin are two Food and Drug Administration (FDA)-approved YAP inhibitors. Dobutamine is commonly used for septic shock and heart failure,^94,95 and verteporfin for cancer, age-related macular degeneration (AMD), central serous chorioretinopathy (CSC), ocular histoplasmosis syndrome, and pathologic myopia.^96–98 In vitro experiment revealed that dobutamine and verteporfin could induce megakaryocytic cell maturation and generate thrombopoiesis.⁹⁹ Dobutamine phosphorylates YAP, inhibits YAP translocation into the nucleus, and interacts with its target genes.⁹⁹ Verteporfin inhibits the interaction between YAP and transcriptional enhanced associate domain (TEAD) and downregulates the expression of their downstream target genes.⁹⁹ Treatment with dobutamine or verteporfin on MEG-01 cells results in an increasing level of MKs maturation, differentiation, and production of platelets, and their thrombopoiesis-stimulating effects may be related to upregulation of Fli-1,⁹⁹ indicating this group of agents’ potential effect in the treatment of thrombocytopenia.

Natural agents that promote platelet production

Ingenol

Ingenol is a natural product with potential effect in the treatment of warts, skin keratoses, and cancers by activating PKC-α/δ.^100,101 Drug screening found that ingenol could promote differentiating MK and producing platelet. Specifically, it could stimulate differentiation of MK in K562 and HEL cells, and accelerated thrombopoiesis in RIT mice model, as an increasing platelet counts and MKs in BM and spleen was seen.¹⁰² RNA-sequencing revealed that the thrombopoiesis-stimulating process of ingenol may foster through PI3K-Akt signaling.¹⁰² Especially, using c-MPL knock-out mice or deleting THPO or c-MPL by their neutralizing antibodies in K562 cells could not suppress the thrombopoietic effect of ingenol, indicating that ingenol may be a THPO-independent thrombopoiesis-stimulating agent, which provides a potential medicine for thrombocytopenia treatment.¹⁰²

3,3′-di-O-methylellagic acid 4′-glucoside

3,3′-di-O-methylellagic acid 4′-glucoside (DMAG), a natural ellagic acid derived from Sanguisorba officinalis L. (SOL), has potential antioxidant and hepatoprotective effect.¹⁰³ DMAG is found to have thrombopoiesis-stimulating effect as increasing differentiation of MKs was observed in the HEL cells with DMAG treatment.¹⁰⁴ According to Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses, molecular docking simulation, molecular dynamics simulation, and western blot analysis in the human erythroleukemia (HEL) cells, DMAG could bind to TLR2, ITGB3, and ITGA2B and activate the PI3K-Akt signaling pathway.¹⁰⁴ In mice model of RIT, DMAG could increase the number of von Willebrand factor (VWF)-positive MKs and platelets, with no difference in the mean platelet volume (MPV) compared with THPO treatment group.¹⁰⁴ The function of DMAG-induced platelets is assessed by platelet adhesion, platelet aggregation, and tail bleeding time assays, indicating the potential value of DMAG on platelet recovery of thrombocytopenia treatment.¹⁰⁴

Conclusion

Thrombopoiesis is a complex process that offers various targets for different thrombopoiesis-stimulating agents. THPO–MPL is the primary pathway of regulating thrombopoiesis. Targeting directly the THPO–MPL pathway or its downstream pathways, such as JAK/STAT, ERK1/2, and PI3K-Akt signaling, accounts for many thrombopoiesis agents. Meanwhile, other pathways, such as the ERβ-dependent pathway and the Hippo pathway, are significantly effective in thrombopoiesis, and drugs promote thrombopoiesis independent to THPO signaling may reveal novel regulator pathway of platelet development.

Some thrombopoiesis drugs are in clinical treatment for various kinds of thrombocytopenia. THPO-RAs are the most commonly used thrombopoiesis-stimulating agents in the treatment of thrombocytopenia as an upgrade alternative of recombinant THPOs. IL-11-based agents and ATRA might be beneficial to promoting platelet production, but they usually play a role as an assistant medicine in the combined therapy for ITP, with little clinical evidence in the treatment of other kinds of thrombocytopenia. However, the risk of drug-induced thrombocytosis and thrombosis events should be considered when using these drugs in clinical practice.

Further investigations are needed for other thrombopoiesis-stimulating agents which are newly found to promote platelet production and may show potential value in supplementing treating regimens for thrombocytopenia. Some agents discussed above such as neurotransmitters – EPI, NE and DA, hormones – estrogen and rhGH, and melatonin, antipsychotics, such as clozapine, two YAP inhibitors and some natural products, their thrombopoiesis-stimulating effects and potential signaling pathways are investigated. Still, more thrombopoiesis agents are investigated, but the underlying mechanisms are unknown, which worth introducing briefly. Valproic acid (VPA) was found to induce MK polyploidy, maturation, PPF, and PLP formation,⁶⁰ and upregulation of FLI-1 was detected in VPA-treated MEG-01 cells.⁹⁹ The thrombopoietic effect of VPA may be different from most HDACIs that inhibit platelet production and cause thrombocytopenia,^105–107 suggesting the specificity of VPA in stimulating thrombopoiesis. BKT140, a CXCR4 antagonist, could induce the development of MKs and stimulate the production of platelet progenitors, potentially reducing the severity and duration of CIT in mice, however, thrombopoiesis-stimulating effects are not found in other CXCR4 inhibitors.¹⁰⁸ Traditional drug for ITP treatment, such as vinca alkaloids (VAs), may have thrombopoiesis-stimulating effects apart from commonly known immunosuppressive effect, according to animal experiment in the rat.^109–111 The development of novel drug screening models has become an effective way for investigating thrombopoiesis-stimulating agents. Using the b1-tubulin reporter line with imMKCL-derived iPSCs, two agents, Wnt-C59 and TCS-359, which promote MK maturation and PLP release, were successfully found.¹¹² After investigation, figuring out the definite mechanism of their thrombopoietic effect are vital for developing novel medicine in clinical treatment of thrombocytopenia.

The future of thrombocytopenia treatment is encouraging for many thrombopoiesis-stimulating agents which are in clinical trials or early investigations. Thrombopoiesis-stimulating agents could deal with different types of thrombocytopenia, such as ITP, CIT, CLD, and IT, according to clinical evidence. Another large amount of thrombopoiesis-stimulating agents is not yet in clinical use, but research evidence has proved their potential effects in thrombocytopenia treatment to a certain extent. Most agents play their effects through THPO–MPL or its downstream signaling pathways, and some novel pathways regulating platelet development should be highly regarded. The development of thrombopoiesis-stimulating agents may enrich the pharmacological armamentarium for medical treatment of thrombocytopenia.

Footnotes

Acknowledgements

The authors thank Panpan Meng, a PhD student, for her effective comments on article organizing and writing.

Declarations

ORCID iDs

Lejun Huang

Jianxuan Xu

Huaying Zhang

Qing Lin

References

Lefrancais

Ortiz-Munoz

Caudrillier

, et al. The lung is a site of platelet biogenesis and a reservoir for haematopoietic progenitors. Nature 2017; 544: 105–109.

Sun

Jin

, et al. Single-cell analysis of ploidy and the transcriptome reveals functional and spatial divergency in murine megakaryopoiesis. Blood 2021; 138: 1211–1224.

Repsold

Joubert

. Platelet function, role in thrombosis, inflammation, and consequences in chronic myeloproliferative disorders. Cells 2021; 10: 3034.

Yang

Luan

Shen

, et al. Developments in the production of platelets from stem cells (Review). Mol Med Rep 2021; 23: 7.

Deutsch

Tomer

. Advances in megakaryocytopoiesis and thrombopoiesis: from bench to bedside. Br J Haematol 2013; 161: 778–793.

Comazzetto

Shen

Morrison

. Niches that regulate stem cells and hematopoiesis in adult bone marrow. Dev Cell 2021; 56: 1848–1860.

Kuter

. The biology of thrombopoietin and thrombopoietin receptor agonists. Int J Hematol 2013; 98: 10–23.

Gauer

Whitaker

. Thrombocytopenia: evaluation and management. Am Fam Physician 2022; 106: 288–298.

Kobos

Bussel

. Overview of thrombopoietic agents in the treatment of thrombocytopenia. Clin Lymphoma Myeloma 2008; 8: 33–43.

10.

Wang

Fang

Huang

, et al. Recombinant human thrombopoietin (rh-TPO) for the prevention of severe thrombocytopenia induced by high-dose cytarabine: a prospective, randomized, self-controlled study. Leuk Lymphoma 2018; 59: 2821–2828.

11.

Vadhan-Raj

Verschraegen

Bueso-Ramos

, et al. Recombinant human thrombopoietin attenuates carboplatin-induced severe thrombocytopenia and the need for platelet transfusions in patients with gynecologic cancer. Ann Intern Med 2000; 132: 364–368.

12.

Geissler

Yin

Ganser

, et al. Prior and concurrent administration of recombinant human megakaryocyte growth and development factor in patients receiving consolidation chemotherapy for de novo acute myeloid leukemia – a randomized, placebo-controlled, double-blind safety and efficacy study. Ann Hematol 2003; 82: 677–683.

13.

Basser

Rasko

Clarke

, et al. Randomized, blinded, placebo-controlled phase I trial of pegylated recombinant human megakaryocyte growth and development factor with filgrastim after dose-intensive chemotherapy in patients with advanced cancer. Blood 1997; 89: 3118–3128.

14.

Rodeghiero

Carli

. Beyond immune thrombocytopenia: the evolving role of thrombopoietin receptor agonists. Ann Hematol 2017; 96: 1421–1434.

15.

Ghanima

Cooper

Rodeghiero

, et al. Thrombopoietin receptor agonists: ten years later. Haematologica 2019; 104: 1112–1123.

16.

Gilreath

Bubalo

. Thrombopoietin receptor agonists (TPO-RAs): drug class considerations for pharmacists. Drugs 2021; 81: 1285–1305.

17.

Bastida

Gonzalez-Porras

Rivera

, et al. Role of thrombopoietin receptor agonists in inherited thrombocytopenia. Int J Mol Sci 2021; 22: 4330.

18.

Bowers

Mytych

Lawrence

, et al. Assessment of romiplostim immunogenicity in pediatric patients in clinical trials and in a global postmarketing registry. Blood Adv 2021; 5: 4969–4979.

19.

Kuter

Newland

Chong

, et al. Romiplostim in adult patients with newly diagnosed or persistent immune thrombocytopenia (ITP) for up to 1 year and in those with chronic ITP for more than 1 year: a subgroup analysis of integrated data from completed romiplostim studies. Br J Haematol 2019; 185: 503–513.

20.

Christakopoulos

DeFor

Hage

, et al. Phase I dose-finding, safety, and tolerability trial of romiplostim to improve platelet recovery after UCB transplantation. Transplant Cell Ther 2021; 27: 497.e1–497.e6.

21.

Bussel

Soff

Balduzzi

, et al. A review of romiplostim mechanism of action and clinical applicability. Drug Des Devel Ther 2021; 15: 2243–2268.

22.

Yamanouchi

Hato

Kunishima

, et al. A novel MYH9 mutation in a patient with MYH9 disorders and platelet size-specific effect of romiplostim on macrothrombocytopenia. Ann Hematol 2015; 94: 1599–1600.

23.

Erickson-Miller

Delorme

Tian

, et al. Preclinical activity of eltrombopag (SB-497115), an oral, nonpeptide thrombopoietin receptor agonist. Stem Cells 2009; 27: 424–430.

24.

Di Buduo

Currao

Pecci

, et al. Revealing eltrombopag’s promotion of human megakaryopoiesis through AKT/ERK-dependent pathway activation. Haematologica 2016; 101: 1479–1488.

25.

Wong

Majewska

Tomiyama

. Management of primary immune thrombocytopenia in a real-world setting in Japan: eltrombopag versus corticosteroids. Int J Hematol 2021; 114: 152–163.

26.

Zaninetti

Gresele

Bertomoro

, et al. Eltrombopag for the treatment of inherited thrombocytopenias: a phase II clinical trial. Haematologica 2020; 105: 820–828.

27.

Ahmed

Bashir

Bassett

, et al. Eltrombopag for post-transplantation thrombocytopenia: results of phase II randomized, double-blind, placebo-controlled trial. Transplant Cell Ther 2021; 27: 430.e1–430.e7.

28.

Qiu

Liao

Huang

, et al. Eltrombopag as first-line treatment for thrombocytopenia among paediatric patients after allogeneic haematopoietic stem cell transplantation. Br J Clin Pharmacol 2021; 87: 2023–2031.

29.

Zhang

Liu

, et al. A systematic review and meta-analysis of the safety and efficacy of anti-thymocyte globulin combined with eltrombopag in the treatment of severe aplastic anemia. Ann Palliat Med 2021; 10: 5549–5560.

30.

Alo’F

Zangrilli

Cupelli

, et al. In vitro effect of eltrombopag alone and in combination with azacitidine on megakaryopoiesis in patients with myelodysplastic syndrome. Platelets 2021; 32: 378–382.

31.

Virk

Kuter

Al-Samkari

. An evaluation of avatrombopag for the treatment of thrombocytopenia. Expert Opin Pharmacother 2021; 22: 273–280.

32.

Kuter

. The structure, function, and clinical use of the thrombopoietin receptor agonist avatrombopag. Blood Rev 2022; 53: 100909.

33.

Al-Samkari

Nagalla

. Efficacy and safety evaluation of avatrombopag in immune thrombocytopenia: analyses of a phase III study and long-term extension. Platelets 2022; 33: 257–264.

34.

Markham

. Avatrombopag: a review in thrombocytopenia. Drugs 2021; 81: 1905–1913.

35.

Kim

. Lusutrombopag: first global approval. Drugs 2016; 76: 155–158.

36.

Tsuji

Kawaratani

Ishida

, et al. Effectiveness of lusutrombopag in patients with mild to moderate thrombocytopenia. Dig Dis 2020; 38: 329–334.

37.

Furuichi

Takeuchi

Uojima

, et al. Lusutrombopag has slightly stronger effects on patients with mild thrombocytopenia compared with those with severe thrombocytopenia: a multicenter propensity score matching study. J Hepatobiliary Pancreat Sci 2022; 29: 439–448.

38.

Alkhouri

Imawari

Izumi

, et al. Lusutrombopag is safe and efficacious for treatment of thrombocytopenia in patients with and without hepatocellular carcinoma. Clin Gastroenterol Hepatol 2020; 18: 2600–2608.

39.

Xie

Zhao

Bao

, et al. Pharmacological characterization of hetrombopag, a novel orally active human thrombopoietin receptor agonist. J Cell Mol Med 2018; 22: 5367–5377.

40.

Mei

Liu

, et al. A multicenter, randomized phase III trial of hetrombopag: a novel thrombopoietin receptor agonist for the treatment of immune thrombocytopenia. J Hematol Oncol 2021; 14: 37.

41.

Paul

Bennett

Calvetti

, et al. Molecular cloning of a cDNA encoding interleukin 11, a stromal cell-derived lymphopoietic and hematopoietic cytokine. Proc Natl Acad Sci USA 1990; 87: 7512–7516.

42.

Weich

Fitzgerald

Wang

, et al. Recombinant human interleukin-11 synergizes with steel factor and interleukin-3 to promote directly the early stages of murine megakaryocyte development in vitro. Blood 2000; 95: 503–509.

43.

Lin

Zhou

Guo

, et al. RhIL-11 treatment normalized Th1/Th2 and T-bet/GATA-3 imbalance in in human immune thrombocytopenic purpura (ITP). Int Immunopharmacol 2016; 38: 40–44.

44.

Chen

Sun

, et al. [Therapeutic efficacy of multigly-cosidorum tripterygium combined with rhIL-11 for immune thrombocytopenia]. Zhongguo Shi Yan Xue Ye Xue Za Zhi 2015; 23: 1400–1403.

45.

Song

, et al. A randomized controlled study of rhTPO and rhIL-11 for the prophylactic treatment of chemotherapy-induced thrombocytopenia in non-small cell lung cancer. J Cancer 2018; 9: 4718–4725.

46.

Wan

Zhang

, et al. Recombinant human interleukin-11 (IL-11) is a protective factor in severe sepsis with thrombocytopenia: a case-control study. Cytokine 2015; 76: 138–143.

47.

Teicher

Chen

Ara

, et al. Interaction of interleukin-11 with cytotoxic therapies in vitro against CEM cells and in vivo against EMT-6 murine mammary carcinoma. Int J Cancer 1996; 67: 864–870.

48.

Berger

Hawley

Lust

, et al. Tyrosine phosphorylation of JAK-TYK kinases in malignant plasma cell lines growth-stimulated by interleukins 6 and 11. Biochem Biophys Res Commun 1994; 202: 596–605.

49.

Suchorska

Dams-Kozlowska

Kazimierczak

, et al. Hyper-interleukin-11 novel designer molecular adjuvant targeting gp130 for whole cell cancer vaccines. Expert Opin Biol Ther 2011; 11: 1555–1567.

50.

Dams-Kozlowska

Kwiatkowska-Borowczyk

Gryska

, et al. Designer cytokine hyper interleukin 11 (H11) is a megakaryopoietic factor. Int J Med Sci 2013; 10: 1157–1165.

51.

Kumar

Biswas

Sharma

, et al. PEGylated IL-11 (BBT-059): a novel radiation countermeasure for hematopoietic acute radiation syndrome. Health Phys 2018; 115: 65–76.

52.

Liang

Qiao

Liu

, et al. Overview of all-trans-retinoic acid (ATRA) and its analogues: structures, activities, and mechanisms in acute promyelocytic leukaemia. Eur J Med Chem 2021; 220: 113451.

53.

Schroeder

Zouboulis

. All-trans-retinoic acid and 13-cis-retinoic acid: pharmacokinetics and biological activity in different cell culture models of human keratinocytes. Horm Metab Res 2007; 39: 136–140.

54.

Melnik

Schmitz

. Are therapeutic effects of antiacne agents mediated by activation of FoxO1 and inhibition of mTORC1. Exp Dermatol 2013; 22: 502–504.

55.

Auci

Egilmez

. Synergy of transforming growth factor beta 1 and all trans retinoic acid in the treatment of inflammatory bowel disease: role of regulatory T cells. J Gastroenterol Pancreatol Liver Disord 2016; 3: 1–8.

56.

Mucida

Park

Kim

, et al. Reciprocal TH17 and regulatory T cell differentiation mediated by retinoic acid. Science 2007; 317: 256–260.

57.

Sakamoto

Koya

Tsukioka

, et al. The effects of all-trans retinoic acid on the induction of oral tolerance in a murine model of bronchial asthma. Int Arch Allergy Immunol 2015; 167: 167–176.

58.

Sakoe

Kirito

, et al. FOXO3A as a key molecule for all-trans retinoic acid-induced granulocytic differentiation and apoptosis in acute promyelocytic leukemia. Blood 2010; 115: 3787–3795.

59.

Visani

Ottaviani

Zauli

, et al. All-trans retinoic acid at low concentration directly stimulates normal adult megakaryocytopoiesis in the presence of thrombopoietin or combined cytokines. Eur J Haematol 1999; 63: 149–153.

60.

Schweinfurth

Hohmann

Deuschle

, et al. Valproic acid and all trans retinoic acid differentially induce megakaryopoiesis and platelet-like particle formation from the megakaryoblastic cell line MEG-01. Platelets 2010; 21: 648–657.

61.

Kinjo

Kizaki

Takayama

, et al. Serum thrombopoietin and erythropoietin levels in patients with acute promyelocytic leukaemia during all-trans retinoic acid treatment. Br J Haematol 1999; 105: 382–387

62.

Huang

Liu

Wang

, et al. All-trans retinoic acid plus high-dose dexamethasone as first-line treatment for patients with newly diagnosed immune thrombocytopenia: a multicentre, open-label, randomised, controlled, phase 2 trial. Lancet Haematol 2021; 8: e688–e699.

63.

Liu

Zeng

, et al. All-trans retinoic acid plus low-dose rituximab vs low-dose rituximab in corticosteroid-resistant or relapsed ITP. Blood 2022; 139: 333–342.

64.

Lokhandwala

Barrett

. Cardiovascular dopamine receptors: physiological, pharmacological and therapeutic implications. J Auton Pharmacol 1982; 2: 189–215.

65.

Dreborg

Kim

. The pharmacokinetics of epinephrine/adrenaline autoinjectors. Allergy Asthma Clin Immunol 2021; 17: 25.

66.

Gough

CJR

Nolan

. The role of adrenaline in cardiopulmonary resuscitation. Crit Care 2018; 22: 139.

67.

Ruslan

Baharuddin

Noor

, et al. Norepinephrine in septic shock: a systematic review and meta-analysis. West J Emerg Med 2021; 22: 196–203.

68.

Chen

Shen

, et al. Sympathetic stimulation facilitates thrombopoiesis by promoting megakaryocyte adhesion, migration, and proplatelet formation. Blood 2016; 127: 1024–1035.

69.

Chen

Shen

, et al. Dopamine induces platelet production from megakaryocytes via oxidative stress-mediated signaling pathways. Platelets 2018; 29: 702–708.

70.

Fischer

Haffner-Luntzer

. Interaction between bone and immune cells: implications for postmenopausal osteoporosis. Semin Cell Dev Biol 2022; 123: 14–21.

71.

Pickar

Komm

. Selective estrogen receptor modulators and the combination therapy conjugated estrogens/bazedoxifene: a review of effects on the breast. Post Reprod Health 2015; 21: 112–121.

72.

Gérard

Arnal

Jost

, et al. Profile of estetrol, a promising native estrogen for oral contraception and the relief of climacteric symptoms of menopause. Expert Rev Clin Pharmacol 2022; 15: 121–137.

73.

Bord

Frith

Ireland

, et al. Estrogen stimulates differentiation of megakaryocytes and modulates their expression of estrogen receptors alpha and beta. J Cell Biochem 2004; 92: 249–257.

74.

Nagata

Yoshikawa

Hashimoto

, et al. Proplatelet formation of megakaryocytes is triggered by autocrine-synthesized estradiol. Genes Dev 2003; 17: 2864–2869.

75.

Yang

, et al. Estrogen promotes megakaryocyte polyploidization via estrogen receptor beta-mediated transcription of GATA1. Leukemia 2017; 31: 945–956.

76.

Bord

Vedi

Beavan

, et al. Megakaryocyte population in human bone marrow increases with estrogen treatment: a role in bone remodeling? Bone 2000; 27: 397–401.

77.

Abdollahzade

Majidinia

Babri

. Melatonin: a pleiotropic hormone as a novel potent therapeutic candidate in arsenic toxicity. Mol Biol Rep 2021; 48: 6603–6618.

78.

Xie

Chen

, et al. A review of sleep disorders and melatonin. Neurol Res 2017; 39: 559–565.

79.

Gunata

Parlakpinar

Acet

. Melatonin: a review of its potential functions and effects on neurological diseases. Rev Neurol (Paris) 2020; 176: 148–165.

80.

Talib

Alsayed

Abuawad

, et al. Melatonin in cancer treatment: current knowledge and future opportunities. Molecules 2021; 26: 2506.

81.

Imenshahidi

Karimi

Hosseinzadeh

. Effects of melatonin on cardiovascular risk factors and metabolic syndrome: a comprehensive review. Naunyn Schmiedebergs Arch Pharmacol 2020; 393: 521–536.

82.

Chen

, et al. Insight into the roles of melatonin in bone tissue and bonerelated diseases (Review). Int J Mol Med 2021; 47: 82.

83.

Chen

Wang

, et al. Melatonin enhances thrombopoiesis through ERK1/2 and Akt activation orchestrated by dual adaptor for phosphotyrosine and 3-phosphoinositides. J Pineal Res 2020; 68: e12637.

84.

Yang

Chen

, et al. Melatonin protects against apoptosis of megakaryocytic cells via its receptors and the AKT/mitochondrial/caspase pathway. Aging (Albany NY) 2020; 12: 13633–13646.

85.

Esmaeili

Nassiri Toosi

Taher

, et al. Melatonin effect on platelet count in patients with liver disease. Gastroenterol Hepatol Bed Bench 2021; 14: 356–361.

86.

van Bunderen

Olsson

. Growth hormone deficiency and replacement therapy in adults: impact on survival. Rev Endocr Metab Disord 2021; 22: 125–133.

87.

Wang

Shen

, et al. hGH promotes megakaryocyte differentiation and exerts a complementary effect with c-Mpl ligands on thrombopoiesis. Blood 2014; 123: 2250–2260.

88.

Vickers

Ramineni

Malacova

, et al. Risk factors for clozapine-induced myocarditis and cardiomyopathy: a systematic review and meta-analysis. Acta Psychiatr Scand 2022; 145: 442–455.

89.

Paribello

Manchia

Zedda

, et al. Leukocytosis associated with clozapine treatment: a case series and systematic review of the literature. Medicina (Kaunas) 2021; 57: 816.

90.

Nielsen

Correll

Manu

, et al. Termination of clozapine treatment due to medical reasons: when is it warranted and how can it be avoided? J Clin Psychiatry 2013; 74: 603–613; quiz 613.

91.

Lee

Takeuchi

Fervaha

, et al. The effect of clozapine on hematological indices: a 1-year follow-up study. J Clin Psychopharmacol 2015; 35: 510–516.

92.

Sui

Tsuji

Ebihara

, et al. Soluble interleukin-6 (IL-6) receptor with IL-6 stimulates megakaryopoiesis from human CD34(+) cells through glycoprotein (gp)130 signaling. Blood 1999; 93: 2525–2532.

93.

Lorthongpanich

Jiamvoraphong

Supraditaporn

, et al. The Hippo pathway regulates human megakaryocytic differentiation. Thromb Haemost 2017; 117: 116–126.

94.

Nadeem

Sockanathan

Singh

, et al. Impact of dobutamine in patients with septic shock: a meta-regression analysis. Am J Ther 2017; 24: e333–e346.

95.

Tacon

McCaffrey

Delaney

. Dobutamine for patients with severe heart failure: a systematic review and meta-analysis of randomised controlled trials. Intensive Care Med 2012; 38: 359–367.

96.

Soubrane

Bressler

. Treatment of subfoveal choroidal neovascularisation in age related macular degeneration: focus on clinical application of verteporfin photodynamic therapy. Br J Ophthalmol 2001; 85: 483–495.

97.

Bakri

Kaiser

. Verteporfin ocular photodynamic therapy. Expert Opin Pharmacother 2004; 5: 195–203.

98.

Brodowska

Al-Moujahed

Marmalidou

, et al. The clinically used photosensitizer Verteporfin (VP) inhibits YAP-TEAD and human retinoblastoma cell growth in vitro without light activation. Exp Eye Res 2014; 124: 67–73.

99.

Lorthongpanich

Jiamvoraphong

Klaihmon

, et al. Effect of YAP/TAZ on megakaryocyte differentiation and platelet production. Biosci Rep 2020; 40: BSR20201780.

100.

Jubert-Esteve

Del Pozo-Hernando

Izquierdo-Herce

, et al. Quality of life and side effects in patients with actinic keratosis treated with ingenol mebutate: a pilot study. Actas Dermosifiliogr 2015; 106: 644–650.

101.

Ersvaer

Kittang

Hampson

, et al. The protein kinase C agonist PEP005 (ingenol 3-angelate) in the treatment of human cancer: a balance between efficacy and toxicity. Toxins (Basel) 2010; 2: 174–194.

102.

Wang

Zhang

Liu

, et al. Discovery of a novel megakaryopoiesis enhancer, ingenol, promoting thrombopoiesis through PI3K-Akt signaling independent of thrombopoietin. Pharmacol Res 2022; 177: 106096.

103.

Ito

Shimura

Watanabe

, et al. Hepatoprotective compounds from Canarium album and Euphorbia nematocypha. Chem Pharm Bull (Tokyo) 1990; 38: 2201–2203.

104.

Lin

Zeng

Liu

, et al. DMAG, a novel countermeasure for the treatment of thrombocytopenia. Mol Med 2021; 27: 149.

105.

Bishton

Harrison

Martin

, et al. Deciphering the molecular and biologic processes that mediate histone deacetylase inhibitor-induced thrombocytopenia. Blood 2011; 117: 3658–3668.

106.

Matsuoka

Unami

Fujimura

, et al. Mechanisms of HDAC inhibitor-induced thrombocytopenia. Eur J Pharmacol 2007; 571: 88–96.

107.

Ali

Bluteau

Messaoudi

, et al. Thrombocytopenia induced by the histone deacetylase inhibitor abexinostat involves p53-dependent and -independent mechanisms. Cell Death Dis 2013; 4: e738.

108.

Abraham

Weiss

Wald

, et al. Sequential administration of the high affinity CXCR4 antagonist BKT140 promotes megakaryopoiesis and platelet production. Br J Haematol 2013; 163: 248–259.

109.

Ebbe

Howard

Phalen

, et al. Effects of vincristine on normal and stimulated megakaryocytopoiesis in the rat. Br J Haematol 1975; 29: 593–603.

110.

Gralla

Raphael

Golbey

, et al. Phase II evaluation of vindesine in patients with non-small cell carcinoma of the lung. Cancer Treat Rep 1979; 63: 1343–1346

111.

Feliu

Cervantes

Blade

, et al. Thrombocytosis in quiescent chronic granulocytic leukaemia after vincristine and 6-mercaptopurine therapy. Acta Haematol 1980; 64: 224–226.

112.

Seo

Chen

Hashimoto

, et al. A beta1-tubulin-based megakaryocyte maturation reporter system identifies novel drugs that promote platelet production. Blood Adv 2018; 2: 2262–2272.