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Abstract

As drug development becomes a long and demanding process, it might also become a barrier to medical progress. Drug safety concerns are responsible for many of the resources consumed in launching a new drug. Despite the money and time expended on it, a significant number of drugs are withdrawn years or decades after being in the market. Cardiovascular toxicity is one of the major reasons for those late withdrawals, meaning that many patients are exposed to unexpected serious cardiovascular risks. It seems that current methods to assess cardiovascular safety are imperfect, so new approaches to avoid the exposure to those undesirable effects are quite necessary. Endothelial dysfunction is the earliest detectable pathophysiological abnormality, which leads to the development of atherosclerosis, and it is also an independent predictor for major cardiovascular events. Endothelial toxicity might be the culprit of the cardiovascular adverse effects observed with a significant number of drugs. In this article, we suggest the regular inclusion of the best validated and less invasive endothelial function tests in the clinical phases of drug development in order to facilitate the development of drugs with safer cardiovascular profiles.

Keywords

atherosclerosis cardiac pharmacology endothelium proarrhythmic effects

The Current Problem of Drug Development

The process of drug development is complex, time consuming, and costly. After a preclinical phase—which could take more than 4 years—the approval of an investigational new drug application to the Food and Drug Administration (FDA) is required, and only then the clinical phases I, II, and III are ready to be started. Following their completion, FDA can approve or reject the new drug or can also request for more studies before a decision. The entire process can rarely be completed in less than 12 years.¹ Moreover, for every 5000 compounds that initiated the preclinical phase, only one will reach the market,² and the current cost of developing a new drug is continuously growing, nowadays, it is estimated to exceed US$990 million.³

A negative consequence of this challenging path has been the significant decline in the number of new drugs submitted to the FDA, not in parallel with the increased number of relevant biomedical discoveries. This wide gap between basic research and clinical application could impede innovation and limit the number of therapies available for several diseases.⁴ A concern about this problem has arisen, and some strategies to improve this critical path from laboratory concept to commercial product are being developed. In the case of medical products, performance is evaluated as safety and effectiveness, so one of these requests is for creating new tools to demonstrate them in a more accurate, faster, and lower-cost approach.^4
–6

Toxicity in Drug Development

In the past, adverse pharmacokinetic and bioavailability were responsible for the majority of attrition in drug development; currently, these reasons contribute less than 10%, and the primary causes are the lack of efficacy (30%) and toxicity (30%). These problems are considerable contributors to the elevated cost of the process as tend to be recognized in latter stages (phases II and III) or even after marketing.⁷ In addition, it is known that over 90% of the market withdrawals are due to drug toxicity, which can lead to a huge expenditure of money and time.⁸ For example, it was estimated that the financial and legal cost of withdrawing rofecoxib cost Merck around US$28 billion.⁹

Although safety issues are a cause of delay and discontinuation during the process, and even a possibility of eliminating unnecessarily potential candidates exists, it seems neither practical nor ethical to simply lower the safety standards, as some people have proposed. We expect marketed drugs to have a well-understood safety profile and a positive benefit/risk balance. However, despite the major relevance of safety assessment, there have been little changes over the years in the traditional tools used for it.^4
–6 But even more, some concerns are present regarding a higher likelihood, compared to previous decades, of unanticipated safety problems once the drug is approved¹⁰ and about an inexplicable deferral in removing those drugs from the market following the detection of severe side effects.^11,12

The determinants for these safety deficits are diverse and not unique. Among the reasons, we could include the lack of specificity to predict the adverse effects in humans that classical animal toxicology may have; the narrow spectrum of patients’ profiles enrolled in clinical phases that can differ importantly from the population that will receive the treatment after the approval; the incapacity to detect during the short follow-up of clinical trials those side effects that appear in a very late stage; and the fact that serious adverse drug reactions (ADRs) are often so rare that a huge number of individuals are required to identify them.^4,5,8

Cardiovascular Safety

The high number of treatments developed for cardiovascular diseases may also have undesirable negative effects on the same anatomy where they act. Besides, the cardiovascular system has proven to be particularly sensitive to a large variety of interaction with drugs prescribed for a different therapeutic indication. In result, the cardiovascular system is the most frequent sites of ADRs, and cardiovascular safety is the major cause for drug discontinuation at all stages of drug development in the United States.¹³

Unfortunately, its leadership does not seem to have changed during the last decades. Some years ago, Lasser et al¹⁴ analyzed the period from 1975 to 2000 and found 81 major changes to drug labeling in the Physicians’ Desk Reference, including the addition of one or more black box warnings or drug withdrawal. Cardiovascular (21%) and hepatic (19%) toxicities were the main culprits of those changes. Afterward, Schuster et al⁸ studied the reasons for drug withdrawals from European and American markets between 1992 and 2002. They collected a total of 16 drugs withdrawals, 94% of them due to safety problems. Again cardiovascular (40%) and hepatic (27%) toxicities were the principal contributors for market discontinuation. And more recently, concordant data coming from 2 pharmaceutical companies (DuPont-Merck and Bristol-Myers-Squibb) during 1993 to 2006¹⁵ describe how the most frequent organs or tissues affected by toxicity are the cardiovascular system (27.3%) and the liver (14.8%).

Mechanisms and Evaluation of Cardiovascular Toxicity

A chemical compound may impair the cardiovascular system performance through 3 particular mechanisms: inducing direct myocardial injury, promoting proarrhythmic changes, and/or altering the vascular integrity and tone. The consequences of these insults depend on both the drug (type, dose, and time of exposure) and the patient (age, gender, race, healthy status, and concomitant treatments). Based on that, their magnitude may be quite diverse: from cardiovascular death or severe irreversible injuries (eg, myocardial or cerebral infarction) to symptomatic or asymptomatic reversible effects (eg, deterioration of ventricular ejection fraction, nonlethal arrhythmias) or pathophysiological alterations that could predispose the patient to future cardiovascular events (eg, hypertension, arrhythmogenic, or thrombogenic substrates).^16,17

The current approaches for assessing cardiovascular safety of new drugs, and their particular limitations, have been recently and extensively reviewed in the literature. As a brief summary, the direct drug-induced myocardial injury can be evaluated from the examination of isolated cultured cardiomyocytes or histopathological tissue samples from animals and with the use of different biomarkers (troponins, natriuretic peptides), imaging (echocardiography, magnetic resonance imaging), and invasive techniques (hemodynamic catheterization).^13,16
–18 The proarrhythmic risk, which has received great attention in last years,^19,20 has been traditionally estimated with the functional evaluation of the potassium channel (I_Kr) responsible for most drug-related long QT syndromes (Human ether-a-go-go-related gene (HeRG) assay), with the study of action potentials in isolated cardiac tissues, and with continuous monitorization of arrhythmias and electrocardiographic intervals (electrocardiograms, telemetry); however, more recently a new cardiac proarrhythmia safety paradigm has been proposed, it is labeled the “Comprehensive In vitro Proarrhythmia Assay” (CiPA) and includes in silico predictive modeling of cellular electrophysiological effects.²¹

But, as previously recognized,^13,17,22 little efforts (eg, lipid profile, inflammatory markers, and blood pressure monitoring) have been done to analyze the drug-related pathophysiological alterations that could produce middle long-term effects in the cardiovascular system. Most of the interest has been exclusively focused on the acute and proarrhythmic consequences of the new compounds, especially their risk of QT prolongation.

The Present of Cardiovascular Toxicity

We have performed a search for the marketed drugs withdrawn by the FDA during a 5-year period (2005-2010).²³ To our knowledge, there have been at least 5 withdrawals related to an associated increased cardiovascular risk (Table 1). In addition, 12 safety alerts concerning increased cardiovascular risk associated with the use of various compounds have been published in the same period (Table 2).

Table 1.

Drugs Withdrawn From the Market Based on Cardiovascular Safety Concerns Between 2005 and 2010.

Drug Name	Brand Name	Company	Approval Date	Drug Use	Withdrawal Date	Withdrawal Reason
Propoxyphene	Darvon	Xanodyne Pharm	August 16, 1957	Pain	November 19, 2010	Increased risk of abnormal heart rhythms (PR and QT prolongation)²⁴
Sibutramine	Meridia	Abbott	November 22, 1997	Obesity	October 08, 2010	Increased risk of heart attack and stroke²⁵
Pergolide	Permax	Lilly	December 30, 1988	Parkinson disease	March 29, 2007	Increased risk of heart valve disease²⁶
Tegaserod	Zelnorm	Novartis	July 24, 2002	Irritable bowel syndrome	March 30, 2007	Increased incidence of cardiovascular ischemic events²⁷
Valdecoxib	Bextra	Pfizer	November 16, 2001	Pain	April 07, 2005	Increased risk of cardiovascular events²⁸

Table 2.

Safety Alerts Concerning Cardiovascular Risk Published Between 2005 and 2010.

Drug Name	Brand Name	Company	Approval Date	Drug Use	Safety Alert Date	Safety Alert Reason
Dolasetron	Anzemet	Sanofi-Aventis	November 09, 1997	Nausea, vomiting	December 17, 2010	Increased risk of QT prolongation and torsades de pointes²⁹
Saquinavir	Invirase	Roche	December 06, 1995	HIV	October 21, 2010	Potential change in the electrical activity of the heart (PR, QT)³⁰
GnRH Agonists	Lupron	Abbott	April 09, 1985	Prostate cancer	October 20, 2010	Increased risk of diabetes and certain cardiovascular diseases (heart attack, sudden cardiac death, stroke)³¹
	Eligard	Sanofi-Aventis	January 23, 2002
	Trelstar	Watson	May 15, 2000
	Zoladex	AstraZeneca	December 29, 1989
Rosiglitazone	Avandia	GSK	May 25, 1999	Diabetes mellitus	September 23, 2010	Increased risk of cardiovascular events, such as heart attack and stroke³²
Fosamprenavir	Lexiva	GSK	October 20, 2003	HIV	December 03, 2009	Increased risk of myocardial infarction and dyslipidemia³³
Haloperidol	Haldol	Johnson and Johnson	April 12, 1967	Agitation	September 17, 2007	Increased risk of QT prolongation and torsades de pointes³⁴
Thiazolidinediones	Avandia Actos	GSK Takeda	May 25, 1999	Diabetes mellitus	August 14, 2007	May cause or exacerbate heart failure^35,36
Thiazolidinediones	Avandia Actos	GSK Takeda	July 15, 1999	Diabetes mellitus	August 14, 2007	May cause or exacerbate heart failure^35,36
Methadone	Dolophine	Roxane	August 13, 1947	Pain	November 27, 2006	Increased risk of QT prolongation and torsades de pointes³⁷
Imatinib	Gleevec	Novartis	May 10, 2001	Chronic myeloid leukemia, gastrointestinal stromal tumors	October 19, 2006	May cause or exacerbate heart failure³⁸
Trastuzumab	Herceptin	Genentech	September 25, 1998	Breast cancer	August 31, 2005	Cardiotoxicity (decreased LVEF, increased risk of heart failure and cardiac death)³⁹
COX-2 selective and nonselective NSAIDs	Various	Various	Various	Pain	April 07, 2005	Increased risk of cardiovascular events⁴⁰
Bevacizumab	Avastin	Genentech	February 26, 2004	Colon and rectum cancer	January 05, 2005	Increased risk of arterial thromboembolic events, including myocardial infarction and angina⁴¹

Abbreviations: COX-2, cyclooxygenase 2; GnRH, gonadotropin-releasing hormone; LVEF, left ventricular ejection fraction; NSAIDs, nonsteroidal anti-inflammatory drugs.

In agreement with the data reported by Lasser et al,¹⁴ in both tables, we can visualize how the majority of ADRs are discovered years, even decades, after the drugs are on the market. Likewise, it should also be noted that QT prolongation is still one of the most frequent reason for cardiovascular safety alert, but clearly, it is not the only responsible.

Remarkably, as a good demonstration of the uncertain current times, even acetaminophen—which is traditionally considered the drug of choice for pain relief in patients with cardiovascular disease due to its theoretical cardiovascular safety—has recently shown to significantly increase heart rate and blood pressure compared to placebo,⁴² in similarity with many nonsteroidal anti-inflammatory drugs (NSAIDs).⁴³

The relevance of the cardiovascular safety assessment in the process of drug development is never sufficiently highlighted. With the present delays in detecting this toxicity, we are exponentially increasing the cost, being at risk of ending the research in new molecules, and so taking away the hope for thousands of patients. For instance, it has been suggested⁴⁴ that one of the Pfizer reasons to interrupt development of new drugs in cardiovascular area was the results (increased deaths and cardiovascular events) in a phase III clinical trial with torcetrapib.⁴⁵

At this point, it is reasonable to conclude that cardiovascular toxicity continues to be underestimated at drug’s market launch and that current methods to select drugs with a proper cardiovascular safety profile are still inaccurate and insufficient. So, we have an imperative need for new approaches to help us deliver cardiovascular safe drugs at acceptable times and reasonable cost, avoiding patients’ exposure unnecessarily to deleterious cardiovascular ADRs.

Why Testing Endothelial Function?

The endothelium is a large homeostatic organ that plays a major role in cardiovascular physiology and disease. Its structure might be seen as a simple cell monolayer lining the entire vascular lumens, but its functions are much more complex and relevant. Through the synthesis and release of several bioactive substances, primarily but not only nitric oxide (NO), it regulates vascular tone and prevents vessels wall inflammation, smooth muscle cell proliferation, and thrombosis.^46
–48 Several pathological circumstances may induce functional and structural alterations. The resultant endothelial dysfunction involves a systemic disorder that comprises the production of vasoconstricting and prothrombotic factors, the expression of adhesion molecules, and the impairment of the normal repair mechanisms. At present, we have a wide range of invasive and noninvasive methods available to assess this endothelial activation in vivo and in vitro.^48,49

Clinical Relevance of Endothelial Dysfunction and Utility of Endothelial Function Tests

All cardiovascular risk factors, such as diabetes mellitus, hypercholesterolemia, hypertension, obstructive sleep apnea, and smoking,^50

–54 have shown to impair endothelial function. Although contradictory data exist,⁵⁵ it has also been pointed out that endothelial dysfunction might be not only the consequence but even a pathogenetic mechanism for the onset of some of them.^55

–59

Endothelial dysfunction is recognized as one of the factors responsible for initiation and progression of atherosclerosis,^60,61 both for the loss of its protective properties and for the induction of an atherothrombotic substrate.⁶² It is an independent predictor for future major cardiovascular events,⁶³ as it also contributes to destabilize the plaque, by changing its biology and composition,⁶⁴ making it more prone to rupture and thus to acute cardiovascular events.⁶⁵ Besides, heart failure is a casual factor for endothelial dysfunction,⁶⁶ and at the same time, it is linked to worse outcomes and high mortality in patients with heart failure.^67,68

Currently, endothelial tests are being used for several applications. They are excellent approaches for a better understanding of the mechanisms involved in the genesis and progression of many different diseases (ie, atherosclerosis,⁶⁹ erectile dysfunction,⁷⁰ pulmonary hypertension,⁷¹ renal insufficiency,⁷² and migraine⁷³); they are quite helpful in assessing the changes in endothelial function and clinical markers resulting from exercise,⁷⁴ dietary,^75
–77 medical,^78
–80 percutaneous,⁸¹ or surgical interventions⁸²; and one of their most promising advantages is their applicability as clinical diagnostic tool for identifying—at earlier stages—those patients with a high risk of cardiovascular events in order to initiate or intensify the proper treatments.^83,84

Drugs and Endothelium

It has been observed in animals that endothelial function and structure may result in damage by 3 different drug-related mechanisms⁸⁵: (1) direct endothelial cell toxicity, through interactions with molecules expressed on the cell membranes; (2) an increase in blood flow-induced shear stress, generated from prolonged vasodilatation or a marked increment in regional blood flow, and (3) an immune-mediated injury. Following any of them, there is a common endothelial activation—quite similar to that observed with the major cardiovascular risks—that comprises synthesis and release of proinflammatory cytokines, upregulation of adhesion molecules, T-cell and complement activation, and autoantibodies production. All these actions result in vessel wall inflammation, leading to an increase in intimal permeability, membrane damage, intimal hyperplasia, and cell death.⁸⁶

The number of marketed drugs with proven arterial toxicity in animals is not negligible,⁸⁵ and it would be possible that the list of drugs that induce endothelial dysfunction in humans was longer in case we tested all compounds. We will just mention below some of the most noticeable interactions, due to both their recent description and clinical relevance.

One of the particularly controversial topics in recent years^9,11,12 has been the market withdrawal of some cyclooxygenase 2 (COX-2) selective NSAIDs and the FDA safety alert for the rest of COX-2 selective and nonselective NSAIDs due to potential serious adverse cardiovascular events.^28,40,87 Although the exact mechanism by which these drugs increase cardiovascular risk is still not fully understood, today we have more clues about the pathological role of endothelial dysfunction on it.^79,88,89 Cyclooxygenase 2 was thought to be only an inducible enzyme associated with inflammation and pain, but it is easily inducible in endothelial cells by shear stress too.⁹⁰ There, COX-2 produces prostacyclin (PGI₂), which promotes vasorelaxation and inhibits platelets activation. One of the postulated mechanisms is that the inhibition of COX-2 will induce the loss of these endothelial PGI₂ cardioprotective effects, leading to the undesirable cardiovascular effects.⁹¹

In the last 2 decades, oncology is becoming a medical specialty with a highly productive research in new drugs. It is reducing the mortality and morbidity of patients with cancer but at the same time is revealing a large number of cardiovascular ADRs. This fact may limit the use of some of these compounds, given that many of the signaling cascades inhibited in cancerous cells are also necessary for myocardial and vascular cell survival.⁹² The vascular endothelial growth factor (VEGF) is essential for the growth and survival of endothelial cells,⁹³ so anti-VEGF drugs (ie, bevacizumab, lapatinib, sunitinib, and sorafenib) are a good paradigm.⁹⁴

The idea of raising protective high-density lipoprotein seems attractive as it is known to enhance endothelial function.⁹⁵ Therefore, the early termination of a phase III clinical trial with torcetrapib,⁴⁵ a cholesteryl ester transfer protein inhibitor, because of an increased risk of death and cardiac events, was not expected. The real cause of these adverse events is still unclear, but a low increase in the blood pressure and serum aldosterone is unlikely to entirely explain the magnitude of the outcomes.^96,97 The first evidence for torcetrapib-induced endothelial dysfunction in vivo are already published.⁹⁸

In chronic kidney disease, oxidative stress and inflammation are associated with impaired activity of the nuclear 1 factor (erythroid-derived 2)-related factor 2 (Nrf2) transcription factor. Bardoxolone methyl is a potent activator of the Nrf2 pathway and was shown to reduce the serum creatinine concentration. However, significantly increased risks of heart failure and of the composite cardiovascular outcome (nonfatal myocardial infarction, nonfatal stroke, hospitalization for heart failure, or death from cardiovascular causes) prompted termination of a randomized trial in patients with type 2 diabetes and stage 4 chronic kidney disease.⁹⁹ It has been seen that through modulation of the endothelin pathway—a potent vasoconstrictor peptide produced in endothelial cells—bardoxolone methyl may promote acute sodium and volume retention and increase blood pressure in patients with more advanced chronic kidney disease.¹⁰⁰

Moreover, different antipsychotics (haloperidol, risperidone, chlorpromazine, and clozapine) have been recently related to cytotoxic effects and apoptosis of endothelial cells.¹⁰¹ This might be one of the reasons for the significantly increased risk for stroke and coronary artery disease (CAD) observed with the use of second-generation antipsychotics.¹⁰²

Endothelial Function Tests to Be Integrated in Drug Development

It is beyond the scope of our article to provide a description of the tests that are used or that might be applied in the preclinical phases of drug development. Besides, there are significant concerns regarding the uncertain extrapolation of some induced vascular toxic effects observed in animals to humans.^85,86 Thus, we will just focus on detailing the evidence that might support, according to our opinion, the introduction of the best validated and less invasive techniques for the assessment of endothelial function in the clinical phases of drug development.

These techniques may be grouped into 2 categories: those tests that measure the appropriate endothelial response to increased shear stress (flow-mediated dilatation [FMD] and reactive hyperemia peripheral arterial tonometry) and those that evaluate the production of biomarkers of endothelial damage and repair (asymmetric dimethylarginine [ADMA] and endothelial progenitor cells [EPCs]).

The most widely used noninvasive test is the FMD, which consists of the ultrasound measurement of the changes in brachial artery diameter due to the release of endothelial NO in response to the increase in shear stress induced by the inflation and subsequent release of a sphygmomanometer cuff on the distal forearm. This response is depressed in participants with atherosclerosis and cardiovascular risk factors.⁴⁹ The assessment of peripheral endothelial function by FMD is closely related to coronary artery endothelial function.¹⁰³ Flow-mediated dilatation is an independent predictor of cardiovascular events in participants with^104,105 and without⁸³ previous cardiovascular disease.

More recently, another noninvasive technique to assess the peripheral endothelial function has been developed. Reactive hyperemia peripheral arterial tonometry measures the changes in digital pulse volume during a similarly induced reactive hyperemia. In the same way, this digital vasodilatation function is related to multiple traditional cardiovascular risk factors¹⁰⁶ and to coronary microvascular endothelial dysfunction.¹⁰⁷ Reactive hyperemia peripheral arterial tonometry is also an independent predictor of cardiovascular adverse events.⁸⁴

Increased plasma levels of ADMA—an endogenous competitive antagonist of NO synthase that impairs endothelial function—are detected in participants with cardiovascular risk factors and diseases¹⁰⁸ and are related to coronary endothelial dysfunction¹⁰⁹ and decreased branchial FMD responses.¹¹⁰ Elevated ADMA levels are an independent predictor of future major adverse cardiac events¹¹¹ and all-cause mortality.¹¹²

Endothelial function is related to the number of EPCs, as these EPCs are responsible for maintaining endothelial integrity after many of the injuries. An inverse correlation has been shown between cardiovascular risks factors and diseases and the number and function of EPCs.¹¹³ As seen above with the previous 3 tests, low levels of EPCs are also independent predictors of CAD progression¹¹⁴ and worse cardiovascular outcomes.^115,116

Conclusion

In the present article, we have shown that cardiovascular safety remains a key problem in drug development. Despite the approaches currently used to detect the toxicity along this process, patients continue to have severe cardiovascular side effects once the drugs are already in the market. It has been clearly stated that endothelial dysfunction plays a main role in the incidence of future major cardiovascular events and that endothelial function tests are valuable tools to determinate the endothelial morphologic and functional integrity. Our review supports the inclusion of some of these endothelial function tests in the hard process of drug development for avoiding the undesirable consequences of drug-induced endothelial toxicity.

Footnotes

Author’s Contribution

J. Ruiz-García and E. Alegría-Barrero are contributed to conception or design, acquisition, analysis, or interpretation, drafted the manuscript, critically revised the manuscript, gave final approval, agree to be accountable for all aspects of work ensuring integrity and accuracy.

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.

Funding

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

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Cardiovascular Safety in Drug Development

Abstract

Keywords

The Current Problem of Drug Development

Toxicity in Drug Development

Cardiovascular Safety

Mechanisms and Evaluation of Cardiovascular Toxicity

The Present of Cardiovascular Toxicity

Why Testing Endothelial Function?

Clinical Relevance of Endothelial Dysfunction and Utility of Endothelial Function Tests

Drugs and Endothelium

Endothelial Function Tests to Be Integrated in Drug Development

Conclusion

Footnotes

Author’s Contribution

Declaration of Conflicting Interests

Funding

References