Intra-articular Injections of Allogeneic Platelet-Rich Plasma from Responder Patients for the Treatment of Knee Osteoarthritis: A Pilot and Feasibility Clinical Trial

Abstract

Objective

To evaluate the feasibility, safety and efficacy of allogeneic platelet-rich plasma (PRP) from responder donors to treat knee osteoarthritis (KOA) patients who showed negative response to autologous PRP.

Design

This pilot feasibility trial included KOA patients who did not respond to previous autologous PRP treatment. They were treated with intra-articular injections of allogeneic PRP from responder donors. Patients filled out Knee injury and Osteoarthritis Outcome Score (KOOS), Visual Analogue Scale (VAS), and Lequesne Index at baseline, 2, 6, and 12 months. Blood and PRP from donors and patients were analyzed, and a cell proliferation study was carried out.

Results

Of the 16 patients enrolled, 14 completed the study. KOOS pain subscale and VAS showed a significant increase from baseline to 12 months, and the Lequesne Index to 6 months (P < .005). Six patients (42.9%) showed a Minimal Clinically Important Improvement. No adverse reactions to allogeneic PRP were reported. The platelet number between donors and recipients was similar (P > .05) with a platelet concentration factor of 2.5. Donors were significantly younger than patients (P < .05) and presented higher levels of IGF-1 (P < .05). Cell bioactivity showed no differences between patient and donor PRP (P > .05).

Conclusion

The use of allogeneic PRP from donor responders is a feasible and safe treatment for KOA patients who do not respond to autologous PRP. This treatment showed efficacy after 1 year of follow-up, suggesting a valid alternative for these patients, although further research is needed.

EU Clinical Trials Register (https://www.clinicaltrialsregister.eu/). Registration number: 2021-001267-24.

Keywords

knee osteoarthritis platelet-rich plasma growth factors allogenic intra-articular injection

Introduction

Knee osteoarthritis (KOA) is one of the most prevalent pathologies worldwide, with a particular incidence in the elderly population.¹ Due to the aging of the population and current lifestyle habits such as sedentarism and overweight, its prevalence will increase in the coming years. It will pose a challenge not only in the field of health but also in the economic sphere.² The economic burden associated with KOA is also due to the lack of conservative treatments that can slow down the progression of the disease. Thus the ultimate solution for these patients is knee arthroplasty.³ This surgical intervention carries not only inherent risks for the patient, but also increased costs due to the growing number of operations, including revision knee arthroplasties.⁴ Therefore, new and more effective treatments and approaches for this pathology are necessary.

In recent years, orthobiological treatments have become a widespread option for the treatment of KOA.⁵ Among these, platelet-rich plasma (PRP) is one of the most researched and applied, and is being considered as a first-line treatment over other injectable alternatives.⁶ This autologous product is obtained from the patient’s blood to obtain a plasma fraction with a higher platelet concentration than in blood. Its therapeutic action is based on the action of growth factors and other plasma and platelet molecules and elements such as extracellular vesicles.⁷ They modulate the pathophysiological processes characteristic of joint degeneration. Its mechanisms of action include anti-inflammatory effect, analgesic action, lubrication, and modulation of cell populations.⁸ PRP studies also provide evidence as a disease-modifying treatment which affects tissues such as cartilage and synovial membrane, as well as various biomarkers.⁹

However, clinical results indicate that this treatment is subject to great variability due to a number of factors, which can improve or worsen the efficacy of PRP.¹⁰ One of the main factors affecting such variability is the patient’s age, body mass index (BMI), or sex, which could determine their response to the treatment.^10-12 Indeed, numerous studies reveal how the donor’s characteristics influence the PRP formulation¹³ as well as the molecular composition of the plasma.^14-17 Thus, the evolution of this type of biological treatment relies on determining the optimal composition to maximize the clinical response in these patients. Although modulation of this composition currently poses a significant challenge,¹⁸ one option could be to use allogeneic PRP that has better characteristics than autologous PRP. This approach has already been addressed by several authors, yielding positive results without safety issues.^19-22 However, so far, no study has evaluated the possibility of applying allogeneic PRP from donors selected on the basis of their positive response to this treatment.

Bearing this in mind, the aim of this work is to evaluate the feasibility, safety, and efficacy of using allogeneic PRP from responder donors to treat KOA patients who have shown negative response to autologous PRP. The hypothesis of the study is that PRP from responder patients may have characteristics that enhance the clinical response in those who have not previously benefited from their own PRP.

Material and Methods

Study Design

The study was designed as a pilot and feasibility prospective clinical trial to analyze the injections of allogeneic PRP from responder donors for KOA ( Fig. 1 ). It was carried out in accordance with the international standard on clinical trials: Real Decreto 223/2004, Declaration of Helsinki in its latest revised version (Fortaleza, Brazil; 2013), Good Clinical Practice Regulations (International Conference for Harmonization), and the STROBE statement. The study protocol was reviewed and approved by the Ethics Committee of the Basque Country (May 2022) (protocol number: UCA-12-EC/21/ALO, EUDRACT: 2021-001267-24) and sanctioned by The Spanish Agency of Medicines and Medical Devices. All patients provided written informed consent before entry into the clinical trial, which was monitored by BioAraba, part of the public health system.

Figure 1.

Flowchart of the study design.

Patient and Donor Selection

Sixteen patients were assessed for eligibility and enrolled from 2022 to 2023. Patients were considered eligible if they were over 18 years of age, presented with KOA confirmed by radiographic imaging, and had not responded to previous autologous PRP treatments, with symptoms persisting. Table 1 compiles the inclusion and exclusion criteria that patients had to meet in order to participate in this study. Enrolment closed on 13 November, 2023, and the pilot study was completed on 18 November, 2024.

Table 1.

Patient Selection Criteria.

Inclusion criteria	Exclusion criteria
Patients of both sexes over 18 years of age	Bilateral KOA requiring infiltration of both knees
Diagnosed KOA by clinical radiological study	Body mass index >35
Candidates for intra-articular PRP treatment	Previous arthroscopy within the last year
Symptomatology after autologous PRP treatment	Intra-articular hyaluronic acid or corticosteroid infiltration within the last 6 months
Joint pain equal to or greater than 2.5 VAS points	Systemic autoimmune rheumatic disease
Body Mass Index values between 20 and 35	Uncontrolled diabetes mellitus
Possibility for observation during the follow-up	Pregnancy or lactation

KOA = knee osteoarthritis; PRP = platelet-rich plasma; VAS = visual analogue scale.

The blood to obtain allogeneic PRP was collected from 16 donors. They were male and female volunteers over 18 years of age, showing significant improvement in their symptomatology after PRP treatment (responders), meeting the necessary requirements to be a blood donor. They underwent analytical tests to rule out hepatitis B, hepatitis C, HIV I/II, and HTLV I/II, in accordance with applicable regulations. Donors were assigned consecutively, one to each patient. Each participant was assigned a code to establish donor–patient code pairs.

Demographic data including age, sex, BMI, and previous treatment were collected from patients and donors. Patient’s KOA grade was assessed using the Ahlbäck scale.

PRP Preparation

From each donor, 126 mL of blood was withdrawn into fourteen 9-mL tubes containing 3.8% sodium citrate as the anticoagulant. After centrifuging the tubes at 580g for 8 minutes at room temperature (BTI Biotechnology Institute, Vitoria-Gasteiz, Spain), the upper fraction of the plasma column was discarded and the lower 2 mL of plasma from each tube, located just above the sedimented red blood cells, but not including the buffy coat, was collected in four tubes. Three of these contained 8 ml of PRP each (for treatment) while one contained 4 mL (for in vitro analysis). Moreover, a 9-mL tube of blood was also drawn from each patient (non-donors) to obtain PRP in the same manner, for use in the in vitro analysis. After labeling each tube with the corresponding donor and patient codes, they were stored at −80°C until their use in the injections and in vitro study.

Treatment

The treatment involved three intra-articular infiltrations of 8 mL of allogeneic PRP on a weekly basis. Two hours before the injection, the tube containing the 8 mL of allogeneic PRP was thawed to be administered to the corresponding patient. Once thawed, it was activated with CaCl2 prior to infiltration. After evacuating all of the synovial fluid, the PRP was infiltrated intra-articularly through the mid-point area of the patellofemoral region, using a lateral approach in order to reach the joint space after lateralization of the patella.¹²

Clinical Outcomes Evaluation

Patients completed the Knee injury and Osteoarthritis Outcome Score (KOOS), Visual Analogue Scale (VAS), and Lequesne index at baseline, 2, 6, and 12 months after the third injection. They were evaluated by a different physician than the one who applied the treatment to prevent detection bias.

The primary efficacy criterion was a change from baseline in joint pain, measured using the KOOS pain subscale. Success rates were calculated based on a reduction in the pain score of at least 9.3 points from baseline (Minimal Clinically Important Improvement, MCII).²³ Secondary efficacy variables were assessed by changes in the KOOS subscales for symptoms, activities of daily living (ADL), function in sport and recreation (Sport/Rec), and knee-related quality of life (QOL). Patients who opted for surgery before the end of follow-up were classified as non-responders, and their basal values were included to calculate their score at that time-point.

Safety Outcomes

To evaluate the safety of the treatment, all complications and adverse events were assessed and reported during patient visits, including their nature, onset, duration, and severity.

In Vitro Analysis

An in vitro study was also carried out in which blood and PRP samples from each donor and patient were hematological characterized. Samples were analyzed using the Sysmex XS-1000i hematology analyzer (Sysmex, Kobe, Japan). These assays included the analysis of the different cell populations (red blood cells, white blood cells and platelets). The PRP was coded according to the UCS (Universal Coding System) for PRP studies.²⁴ This code indicates blood platelet concentration (N1), PRP platelet concentration (N2), the presence of erythrocytes (N3) and leucocytes (N4), activation (N5), and the activation method (N6).

Following the hematological analysis, the PRP samples were activated with CaCL2 to trigger platelet activation and obtain the platelet lysate with which to perform the characterization and bioactivity assay described below.

Enzyme-linked immunosorbent assay (ELISA) assays were carried out to detect vascular endothelial growth factor (VEGF; DVE00; Bio-techne, Minneapolis, MI, USA), tumor growth factor beta 1 (TGF-β1; DB100C; Bio-techne), insulin-like growth factor 1 (IGF-1; DG100B; Bio-techne), hepatocyte growth factor (HGF; DHG00B; Bio-techne), platelet-derived growth factor AB (PDGF-AB; DHD00C; Bio-techne), epidermal growth factor (EGF; DEG00; Bio-techne); fibroblast growth factor (FGF; DFB50; Bio-techne), and brain-derived neurotrophic factor (BDNF; DBD00; Bio-techne). Technical duplicates were carried out for all plasma samples. All protein levels were measured by absorbance. The ratio between PDGF and IGF1 (PDGF/IGF-1) was used to determine the relationship between platelet factors and plasma factors.

The biological activity of the PRP samples was evaluated with NHDF cells (Lonza, Basel, Switzerland). They were kept in an incubator at 37°C in a 5% CO₂ atmosphere. Cells were grown in fibroblast growth basal medium (FBM, Lonza, Basel, Switzerland) supplemented with human fibroblast growth factor (hFGF-B), insulin, gentamicin sulfate-amphotericin (GA-1000) at 0.1% (v/v), 2% fetal bovine serum (FBS) (Lonza, Basel, Switzerland), and 1% penicillin/Streptomycin (P/S) (Thermo Fisher Scientific, Waltham, USA). Cells were incubated with FBM medium supplemented with 10% PRP in a 96-well cell culture plate (655083; Greiner, Kremsmünster, Austria) at a concentration of 1,000 cells/well, and cellular viability was measured at 24 h, 48 h, 72 h, 96 h, and 120 h. Serum free was used as a negative control, and cellular viability was measured in triplicate for each sample by Realtime-Glo MT Cell Viability Assay (Promega, Fitchburg, USA). In addition, a luminescence reading was performed using a TECAN Infinite 200 PRO plate reader (TECAN, Zurich, Switzerland), where the level of luminescence can be considered proportional to the number of viable cells present in the assay.

Sample Size Calculation

Power analysis was conducted to estimate the minimum sample size needed to achieve 80% power at a 5% level of significance for the primary outcome measures. An effect size of 9.3 points (minimal clinically important change, MIC)²³ with a standard deviation (SD) of 12 points was assumed.²⁵ This analysis suggested that a minimum of 13 patients would be required, expecting a dropout rate of 0.1.

Statistical Analysis

Demographic and medical variables (gender, age, BMI and OA grade) were expressed as the mean, standard deviation, and percent. Comparisons were performed using χ² for categorical data, analysis of variance (ANOVA) for continuous outcomes of three or more groups and Student’s t-test for continuous outcomes of two groups; distribution of the samples was assessed by Saphiro–Wilk’s test. Data were considered statistically significant when P < .05. Statistical analysis was performed with SPSS 20.0 (SPSS, Chicago, IL).

Results

Demographic Characteristics

A total of 16 patients were considered eligible to participate in this study, of which 15 were finally enrolled. Of these, 14 completed the study and one dropped out before the first follow-up by his own decision ( Fig. 2 ).

Figure 2.

Study flowchart.

There was no difference between the patient group and the donor group in terms of gender and BMI (P > .05). In contrast, the mean age of the donor group (38.2 ± 8.3) was significantly lower than that of the patient group (59.9 ± 21.1) (P < .001) ( Table 2 ). Four of the patients (28.6%) had a severe grade of KOA (grades 3/4 on the Ahlbäck scale).

Table 2.

Demographic Characteristics.

Parameter	Patients (n = 14)	Donors (n = 14)	P value
Sex (female) (%)	7 (50)	4 (28.6)	.255
Age (mean ± SD)	59.9 ± 12.1	38.2 ± 8.3	<.001***
BMI (mean ± SD)	25.9 ± 2.3	23.8 ± 2.3	.065

BMI = body mass index; SD = standard deviation.

***

P < .001.

PRP Characteristics

There were no differences in platelet levels between patients (420.5 × 10⁶ platelets/mL ± 113.9) and donors (413.5 × 10⁶ platelets/mL ± 99.3) (P > .05), reaching a concentration factor of 2.5 in both groups, with no leucocytes or erythrocytes. According to the UCS for PRP studies, both PRP used in this study were 14-00-11 ( Table 3 ).²⁴

Table 3.

Characteristics of Platelet Rich-Plasma.

1. PRP preparation
Initial blood volume	32 ml
Anticoagulant	Sodium citrate 3.8% (wt/V)
System	Close
Centrifugation	Yes
Number	1
Speed	580g/8 min
Final PRP volume	8 ml
2. PRP characteristics
PRP Type	14-00-11
MPV	10.1 ± 0.6 fL
Red blood cells	<0.01 × 10⁶/μL
White blood cells	<0.05 × 10⁶/μL
Activation	CaCl₂ (10% wt/vol)
3. Application characteristics
Formulation type	Liquid
Administration route	Intra-articular
Dosage	3 injections on a weekly basis
Volume	IA injection: 8 mL
Dose (range of platelets)	IA injection: 2.5 × 10⁹−4.1 × 10⁹
Tissue	Cartilage, synovium
Pathology	Knee osteoarthritis

PRP = platelet-rich plasma; IA = intra-articular; MPV = mean platelet volume.

Table 4 shows the platelet and molecular characteristics of PRP taken from patients and donors, revealing significantly higher levels of IGF-1 in the donor group (109.3 ng/mL ± 39.8) than in the patient group (79.1 ng/mL ± 25.4).

Table 4.

Platelet-Rich Plasma Characteristics.

Parameter (mean ± SD)	Patients	Donors	P value	95% CI
Platelet count (×10⁶ platelets/mL)	420.5 ± 113.9	413.5 ± 99.3	.855	−75.8 to 90.8
Platelet increase factor	2.5 ± 0.7	2.5 ± 0.5	.924	−0.5 to 0.5
MPV (fL)	10.8 ± 1.0	9.9 ± 0.8	.404	−0.4 to 1.0
IGF-1 (ng/mL)	79.1 ± 25.4	109.3 ± 39.8	.028*	−56.8 to −3.5
HGF (pg/mL)	656.8 ± 140.4	567.6 ± 106.1	.076	−10.1 to 188.4
PDGF (pg/mL)	16,588.2 ± 7,130.2	15,956.4 ± 5,199.9	.796	−4,348.4 to 5,612.1
TGF-β (pg/mL)	47,801.9 ± 12,131.4	50,012.8 ± 11,122.1	.627	−11,458.5 to 7,036.9
VEGF (pg/mL)	215.7 ± 161.4	207.4 ± 146.7	.891	−114.3 to 130.8
BDNF (pg/mL)	37,703.7 ± 12,079.4	30,383.6 ± 8,541.3	.083	1,033.4 to 15,673.5
EGF (pg/mL)	697.9 ± 441.9	657.4 ± 298.6	.784	−260.9 to 341.9
FGF (pg/mL)	20.9 ± 21.6	10.4 ± 10.4	.140	−3.7 to 24.7

MPV = mean platelet volume; SD = standard deviation; CI = confidence interval.

p < .05.

Clinical Evaluation

Six out of 14 (42.9%) cases showed a pain reduction of at least 9.3 points (MCII) from baseline to 12 months, while the KOOS pain subscale score showed a significant increase at 12 months post-treatment (P = .049). In addition, the VAS score decreased significantly at both 6 (P < .046) and 12 months (P < .017), and the Lequesne index was also significantly lower 6 months after treatment (P < .031) ( Fig. 3 ).

Figure 3.

Clinical evolution scores. KOOS (A), VAS (B), and Lequesne index (C) at baseline, 2, 6, and 12 months after treatment. ADL = function in daily living; sport/rec = function in sport and recreation; QOL = quality of life. *P < .05 with respect to basal level.

There were no differences in demographic characteristics between responder and nonresponder patients ( Table 5 ).

Table 5.

Patient Demographic Characteristics at 12-Month Response.

Parameter	Nonresponder	Responder	P value	95% CI
Sex (female) (%)	3 (37.5%)	4 (66.7)	.299	−19.4 to 62.8
Age (mean ± SD)	59.7 ± 11.6	60.1 ± 13.8	.952	−15.2 to 14.4
BMI (mean ± SD)	27.1 ± 2.3	24.2 ± 2.3	.115	−0.8 to 6.5
Severity (Ahlbäck) (n) (%)
Mild/moderate	6 (75)	4 (66.7)	.743	−33.2 to 49.1
Severe	2 (25)	2 (33.3)	.743	−33.2 to 49.1
Multiple previous treatments	5 (62.5)	5 (83.3)	.411	−25.5 to 55.5

BMI = body mass index; SD = standard deviation; CI = confidence interval.

Donor Influence on the Response to PRP

When classifying the donors according to whether or not their PRP had caused a positive response in patients, it was observed that donors of new patient responders had a lower BMI (22.4 ± 0.7) than those whose PRP yielded a negative response (24.9 ± 2.5) (P < .05) ( Table 6 ).

Table 6.

Donor Demographic Characteristics and 12-Month Response.

Parameter	Nonresponder	Responder	P value	95% CI
Sex (female) (%)	1 (12.5)	3 (50)	.139	−9.1 to 70.4
Age (mean ± SD)	37.4 ± 7.3	39.2 ± 9.9	.703	−11.8 to 8.2
BMI (mean ± SD)	24.9 ± 2.5	22.4 ± 0.7	.042*	−0.1 to 4.8

BMI = body mass index; SD = standard deviation; CI = confidence interval.

P < .05.

When analyzing differences in the donor PRP used in positive and negative responder patients, it was found that new responder patients had received PRP with significantly lower levels of TGF-β (P < .05) and BDNF (P < .01). The ratio of platelet molecules to plasma molecules (PDGF/IGF-1) was also lower in patients showing a positive response ( Table 7 ).

Table 7.

Donor Platelet-Rich Plasma Characteristics and 12-Month Response.

Parameter (mean ± SD)	Nonresponder	Responder	P value	95% CI
Platelet count (×10⁶ platelets/mL)	447.0 ± 100.4	367.7 ± 86.8	0.148	−32.4 to 191.1
Platelet increase factor	2.5 ± 0.3	2.3 ± 0.7	0.534	−0.5 to 0.8
MPV (fL)	10.0 ± 0.5	9.7 ± 1.0	0.426	−0.6 to 1.3
IGF-1 (ng/mL)	98.2 ± 24.5	120.4 ± 51.0	0.361	−73.6 to 29.4
HGF (pg/mL)	538.2 ± 118.3	601.9 ± 87.2	0.300	−192.7 to 65.2
PDGF (pg/mL)	18,292.1 ± 4,020.8	13,231.4 ± 5,380.2	0.078	−679.6 to 10,801.0
TGF-β (pg/mL)	56,154.3 ± 6,867.3	42,847.6 ± 11,219.4	0.024 *	2,154.9 to 24,458.6
VEGF (pg/mL)	209.9 ± 144.2	204.5 ± 163.4	0.951	−182.2 to 192.9
BDNF (pg/mL)	35,646.2 ± 5,910.5	24,243.9 ± 7,011.4	0.009 **	3,523.4 to 19,281.3
EGF (pg/mL)	799.2 ± 219.6	492.1 ± 308.9	0.061	−16.2 to 630.3
FGF (pg/mL)	9.1 ± 12.3	7.4 ± 14.4	0.613	−17.4 to 10.8
PDGF/IGF-1	209.4 ± 87.9	118.1 ± 44.1	0.046 *	1.85 to 180.8

MPV = mean platelet volume; SD = standard deviation; CI = confidence interval.

P < .05, **P < .01.

Safety Assessment

Other than injection-related discomfort, no adverse effects related to allogeneic PRP were found in any of the patients.

Viability of Fibroblasts Culture Upon Exposure to PRP

When analyzing the bioactivity of donor and patient PRP, no differences in cell proliferation were observed between the two groups at any time point (P > .05). There were also no significant differences in cell proliferation values between those donors that caused a positive response and those that did not (P > .05; Fig. 4 ).

Figure 4.

Cell proliferation analysis. The viability levels of the cells incubated with PRP from responder donors and non-responder patients (A), and from donors classified according to the generated response (B). RLU = relative light units.

Discussion

This study is the first to date to evaluate the application of allogeneic PRP taken from responder donors in patients with KOA who had not previously responded to autologous PRP. The key findings were the absence of adverse effects, and a clinical improvement in 42.9% of patients at 12 months, with significant improvements in the various Patient-Reported Outcome Measures.

The lack of a control arm with autologous PRP makes it difficult to assess whether repeat cycles using allogeneic PRP improve the effectiveness of repeat cycles using autologous PRP. Previous studies have evaluated the application of further cycles of autologous PRP, showing better and more sustained clinical results.^26–28 Gobbi et al. compared a group of patients who received one cycle of three PRP injections with another group who received a second cycle 1 year after the first cycle. Although there was a significant difference in favor of the two-cycle group at 18 months, these differences were not maintained after 2 years of follow-up.²⁶ Vaquerizo et al.²⁸ also found no difference in pain improvement between a group of patients who received a cycle of three PRP injections versus another group who received two cycles, although there were differences in other parameters. However, these studies did not specifically select nonresponder patients. In contrast, a study conducted by Sánchez et al.¹⁰ found that applying a second cycle of autologous PRP in patients with a poor response resulted in a 22% improvement compared to those who received only one cycle. In line with this, the data in this study suggests that the application of a new cycle of allogeneic PRP from responder donors improves the repeat treatment using autologous PRP by 20% according to the MCII. This parameter gives a more approximate idea of the evolution and clinical significance of the patients, since a statistically significant difference in the scores does not necessarily mean clinical improvement. This parameter gives a more approximate idea of the evolution and clinical significance of the patients, as a statistically significant difference in scores does not necessarily imply a clinical improvement. This analysis is becoming increasingly important and allows a better interpretation of the treatment efficacy.²⁹ However, these promising results should be taken with caution, as a large number of variables could affect the outcome.

First, it should be noted that the PRP used in this study was stored at −80°C from the time it was collected until its application. On one hand, clinical studies in which PRP was also frozen before application have shown that this treatment was still effective.³⁰ On the other hand, laboratory studies suggested a platelet alteration that could affect platelet morphology and growth factor levels, though without affecting their bioactivity.^31,32 It is reasonable to think that the use of frozen/thawed allogeneic PRP might be more effective, although more studies are needed and it would also complicate the process of the treatment.

In relation to clinical and demographic factors, the main difference between donors and patients was age, the former being significantly younger than the latter. This age-dependent effect in PRP treatment has already been described in numerous clinical studies, which show that this therapy is more effective when patients are younger.^10,33,34 This may be due to other processes associated with the patient’s own age, such as the severity of KOA or the duration of the symptoms. However, preclinical studies showed that PRP from younger donors also has a more potent effect than PRP from older donors.^11,35,36 Studies on cell cultures revealed that the use of young PRP presented a greater anti-inflammatory¹¹ effect in addition to inducing a more youthful phenotype in chondrocytes.³⁵ Furthermore, in vivo studies have also confirmed this trend with an enhanced effect on cartilage integrity³⁵ and bone structure.³⁶

This effect may be due to differences in PRP composition as age affects both platelet^37,38 and molecular levels.^11,36 In this study, there were no differences in platelet concentration between donor and patient groups, nor between donor groups that generated a positive response and those that did not. In contrast, there were differences at the molecular level, specifically in IGF-1 levels, which align with other studies showing that this growth factor is at higher levels in young individuals.^11,39,40 Although not statistically significant, this is also true when comparing the PRP of donors who induced a positive response with that of those who induced a negative response, suggesting the importance of this platelet-independent factor in biological processes.⁴¹ This would suggest that, although there is currently a tendency to associate the effectiveness of PRP with the number of platelets,^42,43 there are other extra-platelet elements whose modulation and relationship with platelet-derived factors could be key in the efficacy of this treatment.^44,45 Indeed, in this study, the donors that caused a positive response in patients presented a lower ratio of platelet factors such as PDGF to extra-platelet factors such as IGF-1.

In addition to the composition of the PRP, a patient’s lack of response could also be influenced by their cellular receptors, since the mechanism of this treatment relies on the activation of these receptors by the PRP molecules.⁴⁶ Indeed, in vitro studies have shown that blocking specific cell receptors can alter the effect of PRP.⁴⁷ Thus, it is reasonable to assume that, even if a patient receives a PRP with an optimal combination of molecules, their response may be negative if their cellular receptors are affected. This could partly explain the in vitro results of this work, in which PRP with certain differences in composition displayed similar cellular bioactivity but led to different clinical responses. However, more extensive in vitro studies are required in order to be able to link laboratory results to clinical responses in KOA.⁴⁸

Another influential factor in the response to PRP is the severity of KOA. Several authors have observed that the efficacy of intra-articular PRP infiltrations is lower in the most severe cases.^10,49-51 The application of other approaches in these patients, such as intraosseous infiltrations, may enhance the efficacy of treatment.^52,53 All patients participating in this study, including those with severe pathology, received intra-articular PRP, so the application of intraosseous infiltrations could potentially improve the response in these patients.

This work demonstrates the safety of the application of allogeneic PRP from selected donors, without any adverse effects. Thus, this is a feasible treatment approach for patients in whom the use of their own PRP is contraindicated due to, for instance, certain pathologies.⁶ Although further studies are needed, allogeneic PRP was found to be at least as effective as autologous PRP and, although further studies are needed, the data suggest that it may be a promising alternative for patients who do not respond positively to their own PRP. Here, the use of identical PRP (i.e. one donor or one PRP pool) for all patients could have provided more information on the effectiveness of allogeneic PRP.

The main limitations of this study lie in the nature of pilot clinical trials. The lack of a control group calls for caution when interpreting the conclusions, since ethical concerns prevented us from applying a control treatment (autologous PRP) in non-responder patients whose previous treatments had failed. In addition, the sample size was not large enough to perform more extensive analyses to shed more light on the data obtained. Nonetheless, this work may serve as a starting point in the exploration of the use of allogeneic PRP selected from “ideal” donors for use in particular cases. Thus, next steps such as randomized clinical trials or real evidence studies with a large amount of data and variables, including biological factors of donors and patients will allow a closer approach to finding the most optimal PRP-based treatment for KOA.

Conclusion

Footnotes

Acknowledgements

The authors wish to thank M. B. Sánchez, A. Iriondo, M. Montoya, M. J. Arnaiz, C. Pérez de Arrilucea, B. Porras and LL. Zuloaga for their involvement in the processing of the PRP samples.

ORCID iD

Mikel Sánchez

Ethical Approval and Informed Consent Statements

The study protocol was reviewed and approved by the Ethics Committee of the Basque Country (May 2022) (protocol number: UCA-12-EC/21/ALO, EUDRACT: 2021-001267-24) and sanctioned by The Spanish Agency of Medicines and Medical Devices. All patients provided written informed consent before entry into the clinical trial, which was monitored by BioAraba, part of the public health system.

Author Contributions

M.S. and D.D contributed to the conception of the study. M.S., D.D., J.G., and C.J. contributed to the design of the study. M.S., J.G., C.J., S.G., J.O., L.LdD., N.F., and J.A. contributed to the provision patients and clinical issues. D.D., D.M, J.M., and M.B. contributed to materials and lab issues. M.S., D.D., C.J., and R.A., contributed to analysis and interpretation of the data. M.S., D.D, R.A., J. E-M., and J.G. contributed to drafting, writing, critical revision, and final approval of the article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

The data presented in this study are available within the article. Additional inquiries may be directed to the corresponding authors.

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