Methods, strengths, weaknesses, and limitations of bioequivalence tests with special regard to immunosuppressive drugs

Generic Medicinal Products in Immunosuppressive Therapy-Should It be a Challenge for Therapeutic Drug Monitoring?

ABSTRACT

Immunosuppressants have a narrow therapeutic index (NTIDs). Indisputably cyclosporine, tacrolimus, everolimus, and sirolimus have NTIDs, and only in the case of mycophenolic acid, a scientific discussion has not been yet concluded. Their specificities highlight the implications for generics introduced into the drug market, more precisely, with bioequivalence testing. In the European Union, the European Medicines Agency (EMA) released the "Guideline on the Investigation of Bioequivalence." The bioequivalence (BE) of the generic (tested, T) versus original (reference, R) product should be confirmed by obtaining a 90% confidence interval (CI) for the T:R ratio of each of the 2 decisive pharmacokinetic parameters, namely, the area under the curve (AUC) between 90.00% and 111.11%. A similar approach (90.00%-112.00%) for AUC was adopted by the Canadian Agency for Drugs and Technologies in Health (CADTH) for NTIDs; however, the US Food and Drug Administration is still based on classic acceptance criteria: 90% CI between 80.00% and 125.00% but with special requirements of BE testing. A discussion about long-expected global consensus was performed in this study based on the literature concerning BE testing in the case of NTIDs. The narrow acceptance criteria reduce the potential mean difference in bioavailability between generic and original products by a few percent. To identify this problem, special attention has been paid to switching drugs (generic-generic, original-generic) and therapeutic drug monitoring after conversion (TDM). There is no global consensus on the acceptance criteria for the BE of generic drugs; therefore, consensus and harmonization are strictly necessary. This study presents a review of the generic drug market and its classification by manufacturers, drug agencies, and dates of marketing authorization. Guidelines for TDM optimization (during switching/conversion) have been proposed. Physicians and clinical pharmacists should pay special attention to switching immunosuppressive drugs between original versus generic formulations, and generic versus generic formulations. Patients and their families should be educated on the risks associated with uncontrolled conversion.

Personalized Therapy for Mycophenolate: Consensus Report by the International Association of Therapeutic Drug Monitoring and Clinical Toxicology

ABSTRACT

When mycophenolic acid (MPA) was originally marketed for immunosuppressive therapy, fixed doses were recommended by the manufacturer. Awareness of the potential for a more personalized dosing has led to development of methods to estimate MPA area under the curve based on the measurement of drug concentrations in only a few samples. This approach is feasible in the clinical routine and has proven successful in terms of correlation with outcome. However, the search for superior correlates has continued, and numerous studies in search of biomarkers that could better predict the perfect dosage for the individual patient have been published. As it was considered timely for an updated and comprehensive presentation of consensus on the status for personalized treatment with MPA, this report was prepared following an initiative from members of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology (IATDMCT). Topics included are the criteria for analytics, methods to estimate exposure including pharmacometrics, the potential influence of pharmacogenetics, development of biomarkers, and the practical aspects of implementation of target concentration intervention. For selected topics with sufficient evidence, such as the application of limited sampling strategies for MPA area under the curve, graded recommendations on target ranges are presented. To provide a comprehensive review, this report also includes updates on the status of potential biomarkers including those which may be promising but with a low level of evidence. In view of the fact that there are very few new immunosuppressive drugs under development for the transplant field, it is likely that MPA will continue to be prescribed on a large scale in the upcoming years. Discontinuation of therapy due to adverse effects is relatively common, increasing the risk for late rejections, which may contribute to graft loss. Therefore, the continued search for innovative methods to better personalize MPA dosage is warranted.

Assessment of prescribability and switchability by using multiple bioequivalence assessment approaches

BACKGROUND

In the drug development process, an assessment of bioequivalence is an integral part. For the evaluation of generics against the comparator, average bioequivalence approach is the gold standard method. In the recent past, there were many discussions on whether we have the adequate tool to evaluate generics and thereby drug interchangeability (prescribability and switchability) issue is addressed as average bioequivalence approach just considers population mean. Hence, the alternative approaches like population bioequivalence and individual bioequivalence assessment approaches arise as different variances like inter/ intra-subject variance and subject-by-formulation variance along with population mean are considered.

OBJECTIVES

Methoxsalen, in combination with long-wave UVA radiation, is used in the symptomatic management certain psoriasis. The study was aimed to establish the bioequivalence (BE) of a newly developed methoxsalen capsule (MTX test) with that of a reference methoxsalen capsule (MTX reference) using multiple BE methods (i.e., average [ABE], population [PBE], and individual [IBE]) by utilizing a new LC-MS/MS method.

METHODS

This is an open-label, randomized, balanced, two-treatment, three-period, three-sequence, crossover, single-dose (20 mg, 2 × 10 mg capsules), comparative, oral BE study conducted in 52 healthy, adult males under fasting conditions. Along with various pharmacokinetic (PK) parameters ABE, PBE, and IBE were also determined in the single study.

RESULTS

A non-compartmental model best described the concentration-time data of both MTX test and reference. Both the formulations demonstrated nearly similar values of BE parameters (i.e., AUCo-t, AUC0-∞, Cmax, Tmax, and t1/2). For MTX test, the observed Cmax, AUC0-t, and AUC0-∞ were 125.16±81.53 ng/mL, 313.73±260.86 ng h/mL, and 321.25±271.85 ng h/mL, respectively. For MTX reference, the values were 127.63±71.60 ng/mL, 329.11±252.91 ng h/mL, and 335.48±264.54 ng h/mL, respectively. The bioanalytical method was validated over the concentration range 0.100-100.00ng/mL and the coefficient of determination (r2) was ≥ 0.9991. The sensitivity of the method was 0.100 ng/mL with the accuracy and precision values of 115% and 10.54%, respectively.

CONCLUSION

A single dose of MTX test met the ABE criteria of 80.00% -125.00% for Cmax, AUCo-t, and AUC0-∞, against MTX reference. The study outcome by PBE and IBE approaches proved that MTX Test was bio-inequivalent to MTX reference. Using multiple BE assessment methods in a single BE study is a novel approach and may overcome shortcomings of conventional bioequivalence assessment methods.

Meeting Regulatory Requirements for Drugs with a Narrow Therapeutic Index: Bioequivalence Studies of Generic Once-Daily Tacrolimus

Despite growing clinical confidence in generics and their potential to reduce long-term healthcare costs, the transplant community have had real concerns about the use of generic immunosuppressants. One such immunosuppressant is tacrolimus, a cornerstone of lifelong treatment for patients who have undergone a solid organ transplant. Tacrolimus has a narrow therapeutic index (NTI), giving rise to questions about the potential for clinically relevant altered drug exposure. Its use in transplant patients also gives rise to questions about the most discriminative subject population for bioequivalence studies. The recognised need for stringent criteria to support approval of generic drugs with an NTI led the European Medicines Association and Health Canada to provide detailed information on requirements for bioequivalence studies and introduce tighter bioequivalence limits for these drugs, including tacrolimus. The aim of this article is to illustrate how regulatory guidance is implemented during the clinical development of generic immunosuppressants, using a generic, once-daily prolonged-release formulation of tacrolimus as an example.

Has the Time Come to Employ Population and Individual Bioequivalence for the Evaluation of Generics?

BACKGROUND

Bioequivalence studies are a vital part of drug development. The average bioequivalence approach is the standard method of assessment to conclude whether the generic product is bioequivalent to the innovator product. Of late, debates are on whether the average bioequivalence approach adequately addresses drug interchangeability as it considers only population mean for the evaluation especially when highly variable drug products and narrow therapeutic index drugs are dealt with. Hence, the alternative approaches like population bioequivalence and individual bioequivalence assessment approaches emerge as they consider inter/intra-subject variance and subject- by-formulation variance along with population mean.

OBJECTIVES

The objective of the study was to apply different bioequivalence assessment approaches in a replicate bioequivalence study to evaluate the drug interchangeability.

METHODS

This was an open-label, single-dose, randomized, balanced, two-treatment, three-period, three-sequence, partial replicate crossover bioequivalence study of omeprazole enteric-coated tablet 20 mg conducted on 48 normal healthy subjects under fed conditions. The plasma concentration of omeprazole was analyzed by a validated bioanalytical method to determine the pharmacokinetic and statistical parameters to assess average bioequivalence, population bioequivalence, and individual bioequivalence.

RESULTS

In this study, test formulation was shown to be bio-inequivalent to the reference formulation by average bioequivalence, population bioequivalence, and individual bioequivalence approaches.

CONCLUSION

The outcome of the evaluation clearly states that the bioequivalence outcome of all these approaches are the same. Obviously, it does not mean that these three approaches provide the same outcome though the consideration of variances varies. Certainly, population bioequivalence and individual bioequivalence approach will be more accurate for the assessment of drug interchangeability.

Comparison of clinical effects, trough and peak levels between branded and generic formulation of Cyclosporine in stable psoriatic patients

BACKGROUND

Cyclosporine A (CyA) is a drug for moderate-to-severe psoriasis. Recently, a generic formulation has been approved as bioequivalent to the branded one. The guidelines for the bioequivalence for critical-dose drugs with a narrow therapeutic range, such as CyA, are questionable. Therefore, it is important to assess the clinical outcome and the pharmacokinetics of different formulations in various patient groups. The current literature lacks of this information in dermatology. The primary objective of this prospective study is to investigate the clinical equivalence (in terms of maintenance of clinical effect) between the generic formulation of CyA and its branded one in patients with psoriasis. A secondary objective is to analyze their trough (C<inf>0</inf>) and peak levels (C<inf>2</inf>).

METHODS

Twenty patients with stable psoriasis under treatment with the branded CyA were monitored in terms of clinical efficacy (psoriasis area severity index, PASI), safety (laboratory values), and their pharmacokinetics utilizing trough (C<inf>0</inf>) and peak plasma concentration (C<inf>2</inf>). The same patients were subsequently shifted to the generic formulation for comparison.

RESULTS

In our sample the efficacy of the two formulations was equal in most cases (P=0.863). A non-significant difference between the C<inf>0</inf> and C<inf>2</inf> of the branded CyA compared to the generic one emerged (respectively P=0.738 and P=0.695).

CONCLUSIONS

The branded and the generic formulations of CyA seem to be not only bioequivalent, but also comparable in terms of clinical efficacy in patients with psoriasis. However, larger samples are required to confirm these findings.

On assessing bioequivalence and interchangeability between generics based on indirect comparisons

As more and more generics become available in the market place, the safety/efficacy concerns may arise as the result of interchangeably use of approved generics. However, bioequivalence assessment for regulatory approval among generics of the innovative drug product is not required. In practice, approved generics are often used interchangeably without any mechanism of safety monitoring. In this article, based on indirect comparisons, we proposed several methods to assessing bioequivalence and interchangeability between generics. The applicability of the methods and the similarity assumptions were discussed, as well as the inappropriateness of directly adopting adjusted indirect comparison to the field of generics' comparison. Besides, some extensions were given to take into consideration the important topics in clinical trials for bioequivalence assessments, for example, multiple comparisons and simultaneously testing bioequivalence among three generics. Extensive simulation studies were conducted to investigate the performances of the proposed methods. The studies of malaria generics and HIV/AIDS generics prequalified by the WHO were used as real examples to demonstrate the use of the methods. Copyright © 2017 John Wiley & Sons, Ltd.

Is There Evidence to Support Brand to Generic Interchange of the Mycophenolic Acid Products?

The uptake of generic immunosuppressants lags comparatively to other drug classes, despite that the Food and Drug Administration (FDA) uses identical bioequivalence standards for all drugs. Transplant societies acknowledge the cost savings associated with generic immunosuppressants and support their use following heart, lung, kidney, or bone marrow transplantation. Seven studies of the pharmacokinetics or clinical efficacy of generic mycophenolate mofetil compared to the innovator product are published; all studies and products were ex-United States. Three studies did not demonstrate any pharmacokinetic differences between generic and innovator products in healthy subjects, achieving FDA bioequivalence requirements. Two studies in renal allograft recipients demonstrated no difference in area under the curves between generic and innovator products, and in one, the maximum concentration (Cmax) fell outside the FDA regulatory range. Two studies revealed no difference in acute organ rejection or graft function in renal allograft recipients. Patient surveys indicate that cost is a barrier to immunosuppressant adherence. Generics present a viable method to reduce costs to payers, patients, and health care systems. Adherence to immunosuppressants is crucial to prevent graft failure. An affordable regimen potentially confers greater adherence. Concerns regarding the presumed inferiority of generic immunosuppressants should be assuaged by regulatory requirements for bioequivalency testing, transplant society position statements, and pharmacokinetic and clinical studies.

Mycophenolate revisited

The patent of mycophenolate mofetil (MMF) has expired, and for enteric-coated mycophenolate sodium (EC-MPS), this will happen in 2017. In the twenty years these drugs have been used, they have become extremely popular. In this review, the reasons for the popularity of mycophenolate are discussed, including the benefits compared to azathioprine. MMF and EC-MPS are therapeutically equivalent. Although neither is considered to be a narrow therapeutic index drug, this should not lead to careless switching between the innovator drug and generic formulations, or between one generic formulation and another. The pipeline of new immunosuppressive drugs is dry, and it is very likely that we will be using mycophenolate for many more years to come as a first-line immunosuppressive drug in our transplant population. Whether or not the development of donor-specific anti-HLA antibodies is related to drug exposure (mycophenolic acid concentrations) remains to be investigated.

Transition from brand to generic tacrolimus is associated with a decrease in trough blood concentration in pediatric heart transplant recipients

There are limited data available on the bioequivalence of generic and brand-name tacrolimus in pediatric and heart transplant patients. We characterized changes in 12-hour trough concentrations and clinical outcomes after transition from brand to generic tacrolimus in pediatric thoracic organ transplant recipients. Patients with a pharmacy-confirmed date of switch between generic and brand tacrolimus were identified, as well as a matched control group that did not switch for comparison. We identified 18 patients with a confirmed date of switch, and in 12 patients that remained on the same dose, trough concentrations were 14% less than when they were on brand (p = 0.037). The average change was -1.15 ± 1.76 ng/mL (p = 0.045). The control group did not experience a change in trough concentration and was different than the switched group (p = 0.005). There were no differences in dosage changes or kidney or liver function. In the year after switch, 24% of patients who were switched to generic experienced a rejection event vs. 18% in the patients on brand. We suggest a strategy of monitoring around the time of transition, and education of the patient/family to notify the care team when changes from brand to generic or between generics occur.

Advances in pharmacotherapy to treat kidney transplant rejection

INTRODUCTION

Current immunosuppressive combination therapy provides excellent prevention of T-cell-mediated rejection following renal transplantation; however, antibody-mediated rejection remains of high concern and accounts for a large number of long-term allograft losses. The recent development of protocol biopsies resulted in the definition of subclinical rejection (SCR), showing histologic evidence for rejection but unremarkable clinical course.

AREAS COVERED

This review describes the current knowledge and evidence of pharmacotherapy to treat kidney allograft rejections and covers SCR treatment options. Each substance is analyzed with regard to its classical indication and further discussed for the treatment of other forms of rejection.

EXPERT OPINION

Despite a lack of randomized trials, early acute T-cell-mediated rejection can be treated effectively in most cases without graft loss. The necessity to treat SCR is currently unclear. Due to a lack of effective therapies, new treatment approaches for antibody-mediated rejection are an urgent medical need to improve long-term outcomes. Future research should aim to better define pathophysiology and histology, stratify risk, and develop rational treatment strategies from randomized controlled trials, in order to establish the value of novel therapies in the arsenal of rejection pharmacotherapy. However, the effective prevention of rejection with minimal side effects still remains the goal in immunosuppression.

Safety and efficacy of the switch to generic mycophenolate mofetil and tacrolimus in heart transplant patients

BACKGROUND

Generic immunosuppressants may offer economic advantages, but their use is still controversial. At our center, 55 heart transplant patients were switched from CellCept(®) to Myfenax Teva(®) (MT) (n = 51, 18% female, 8.1 ± 6.6 yr post-transplantation) and/or Prograf(®) to Tacrolimus Sandoz(®) (TS) (n = 17, 41% female, 6.6 ± 5.8 yr post-transplantation).

METHODS

We conducted an acute monitoring and a retrospective follow-up with regard to safety and efficacy. Acute cellular rejections (ACRs) on endomyocardial biopsies (EMBs) four wk after the MT switch were specifically compared to a matched retrospective control group.

RESULTS

Tacrolimus C0 levels (TS switch) as well as hemoglobin, leukocytes, and thrombocytes (MT switch) did not change (p = NS) during the three wk after each respective switch (8.7 ± 2.9 vs. 8.4 ± 1.9 μg/L, 129.1 ± 12.6 vs. 130.1 ± 12.8 g/L, 6.3 vs. 6.2 × 10(9) /L, and 217.4 ± 56.6 vs. 219.3 ± 61.8 × 10(9) /L, respectively). 0% of the EMBs in the MT switch vs. 3% of the EMBs in the control group showed ACR>grade 1R (p = NS). After six months, survival was 96% (MT switch) and 100% (TS switch), and the frequency of severe ACR was low. Safety parameters measured at the next annual follow-up were also stable following each switch.

CONCLUSION

Switching to MT and/or TS several years after heart transplantation appeared safe in the short-term perspective, showing no detectable changes in tacrolimus C0 levels, safety or efficacy, during an average follow-up of six months.

Intra-patient variability in tacrolimus exposure: causes, consequences for clinical management

Tacrolimus (Tac) is widely used for the prevention of rejection after solid organ transplantation. Finding the optimal balance between effective Tac concentrations and toxicity is a challenge and requires therapeutic drug monitoring. In addition to the well-known inter-patient variability, the clinical use of Tac is also complicated by considerable intra-patient variability (IPV) in Tac exposure. Tac IPV is defined as the amount of fluctuation of whole-blood concentrations over a certain period of time during which the Tac dose remains unchanged. A high IPV in Tac exposure has recently been recognized as a strong risk factor for acute rejection and poor long-term kidney transplantation outcome. In addition to non-adherence, several other factors determine the magnitude of the IPV in Tac exposure. Quantification of IPV is easy and can be easily incorporated into everyday clinical practice as a tool for optimizing transplantation outcomes.

Limustin®, a non-innovator tacrolimus formulation, yields reduced drug exposure in pediatric renal transplant recipients

The aim of this study was to evaluate the bioavailability of two oral tacrolimus formulations, the innovator Prograf(®) and a formulation commercialized in Mexico with the brand name Limustin(®), in children. Stable Mexican pediatric renal transplant recipients received the product authorized by their social security provider, being either Prograf(®) or Limustin(®). At steady state, blood samples were drawn and tacrolimus blood concentration against time curves was constructed. CYP3A5 genotype was also determined. There was no significant difference in dose or in trough concentrations between formulations. However, AUC and Cmax were significantly higher with Prograf(®). The lower tacrolimus bioavailability with Limustin(®) was observed in both expressers and non-expressers of the functional CYP3A5 protein. Dose-normalized AUC values in expressers were 12.7 ± 11.9 and 48.7 ± 20.4 ng·h/mL/mg for Limustin(®) and Prograf(®), whereas in non-expressers, dose-normalized AUC was 54.4 ± 49.1 and 110.4 ± 42.9 ng·h/mL/mg for Limustin(®) and Prograf(®), respectively (p < 0.05). Pharmaceutical quality analysis showed that Limustin(®) dissolution at 120 min was 31.1 ± 6.2% while Prograf(®) dissolution was 100 ± 4.8%. Furthermore, the mean percentage of labeled amount of Limustin(®) and Prograf(®) was 91.0 ± 3.1% and 100.0 ± 0.7%, respectively. Hence, Limustin(®) exhibits pharmaceutical characteristics dissimilar to the innovator that likely explain the reduced tacrolimus exposure in children. We consider Limustin(®) is not adequate for pediatric use.

Bioavailability of a generic of the immunosuppressive agent mycophenolate mofetil in pediatric patients

The use of generic immunosuppressive agents is controversial, especially for the treatment of pediatric patients, as information on the bioavailability of generic immunosuppressants in children is particularly scarce. The aim of the study was to compare the bioavailabilities of two products containing mycophenolate mofetil, the innovator and a generic, in children. Pediatric patients with end-stage renal disease on the waiting list for renal transplantation received a single oral dose of mycophenolate mofetil as either the innovator product (CellCept(®) , Roche) or the generic (Tevacept(®) , Teva Pharmaceuticals). A nine point pharmacokinetic profile was obtained. Mycophenolic acid concentration was quantitated in plasma by HPLC, plasma concentration-against-time curves were constructed, and bioavailability parameters were determined. Pharmaceutical quality analysis of both formulations, including drug content and dissolution profile, was also performed. There were no statistically significant differences between formulations in bioavailability parameters. Interindividual variability was very important, but individual values of AUC, an indicator of the extent of drug absorption, were within the same range for both formulations. The two formulations exhibited similar drug content and dissolution profiles, as well as comparable mycophenolic acid plasma levels in an end-stage renal failure population.

Equivalence and interchangeability of narrow therapeutic index drugs in organ transplantation

The calcineurin inhibitors (CNIs), ciclosporin and tacrolimus, are the mainstay of immunosuppression in solid organ transplantation. Generic formulations of these drugs are now available. With increasing pressure on healthcare budgets and the consequent need to match health expectations to available resources, substitution with a generic product appears an attractive option to reduce costs. Approval of generic products differs from innovator drugs, and narrow therapeutic index drugs (NTIs; including CNIs) bring their own particular considerations. With NTIs, small variations in drug exposure could result in reduced immunosuppression or drug toxicity with potentially adverse effects on patient outcomes. NTIs are subject to stricter regulatory approval versus many other generic drugs. However, different generic formulations may still not necessarily be therapeutically equivalent in individuals, raising the possibility of significant differences in exposure between products. Although regional recommendations vary, many guidelines emphasise the need for NTI drug substitution to be initiated by the transplant physician, thus ensuring careful therapeutic monitoring and reduced negative patient impact. The need for therapeutic monitoring during generic substitution has important implications for the overall costs of generic treatment as these costs have to be factored in to the potential savings made from using generic formulations. The reduced acquisition costs of generic products may not necessarily translate into lower overall healthcare costs. This article examines the issue of equivalence and interchangeability of NTI drugs used in organ transplantation, the implications of the approval process for generic drugs on treatment efficacy and safety, and the effective management of substitutions between products.