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

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.

Recent Advances in Therapeutic Drug Monitoring of Voriconazole, Mycophenolic Acid, and Vancomycin: A Literature Review of Pediatric Studies

The review includes studies dated 2011-2021 presenting the newest information on voriconazole (VCZ), mycophenolic acid (MPA), and vancomycin (VAN) therapeutic drug monitoring (TDM) in children. The need of TDM in pediatric patients has been emphasized by providing the information on the differences in the drugs pharmacokinetics. TDM of VCZ should be mandatory for all pediatric patients with invasive fungal infections (IFIs). Wide inter- and intrapatient variability in VCZ pharmacokinetics cause achieving and maintaining therapeutic concentration during therapy challenging in this population. Demonstrated studies showed, in most cases, VCZ plasma concentrations to be subtherapeutic, despite the updated dosages recommendations. Only repeated TDM can predict drug exposure and individualizing dosing in antifungal therapy in children. In children treated with mycophenolate mofetil (MMF), similarly as in adult patients, the role of TDM for MMF active form, MPA, has not been well established and is undergoing continued debate. Studies on the MPA TDM have been carried out in children after renal transplantation, other organ transplantation such as heart, liver, or intestine, in children after hematopoietic stem cell transplantation or cord blood transplantation, and in children with lupus, nephrotic syndrome, Henoch-Schönlein purpura, and other autoimmune diseases. MPA TDM is based on the area under the concentration-time curve; however, the proposed values differ according to the treatment indication, and other approaches such as pharmacodynamic and pharmacogenetic biomarkers have been proposed. VAN is a bactericidal agent that requires TDM to prevent an acute kidney disease. The particular group of patients is the pediatric one. For this group, the general recommendations of the dosing may not be valid due to the change of the elimination rate and volume of distribution between the subjects. The other factor is the variability among patients that concerns the free fraction of the drug. It may be caused by both the patients' population and sample preconditioning. Although VCZ, MMF, and VAN have been applied in pediatric patients for many years, there are still few issues to be solve regarding TDM of these drugs to ensure safe and effective treatment. Except for pharmacokinetic approach, pharmacodynamics and pharmacogenetics have been more often proposed for TDM.

Important considerations for drugs, nutritional, and herbal supplements in pediatric solid organ transplant recipients

Pediatric transplant recipients are on multiple prescription and non-prescription drugs. Many patients also use dietary, nutritional, and herbal supplements. This manuscript researched formulations of immunosuppressive drugs currently available and presents information on generic immunosuppressive drugs, commonly used non-prescription medications, dietary supplements, and herbal supplements. Immunosuppressive drugs are available in various formulations. Not all formulations are interchangeable. A number of FDA-approved generic formulations are available commercially in the United States. Generally generic formulations produce similar blood concentration vs time profiles compared to brand name products in adults and are considered to be bioequivalent. NSAID should be avoided in transplant patients due to potential drug interactions and increased risk associated with NSAID use; and appropriate doses of acetaminophen should be used for treatment of pain. Over-the-counter medications, such as guaifenesin and dextromethorphan, antihistamine medications, including diphenhydramine, loratadine, cetirizine, and fexofenadine, can be safely used in pediatric solid organ transplant population. Many safe and effective over-the-counter options exist for stool softening and as laxative. Diarrhea can lead to an increase in calcineurin inhibitor levels. Food can alter the absorption of immunosuppressive drugs. Several herbal products can alter immune status of the patients or alter the blood concentration of immunosuppressive drugs or may produce renal or hepatic toxicities and should be avoided in pediatric transplant recipients. It is important to educate pediatric transplant recipients and their families about not only immunosuppressive drug therapy but also about non-prescription drugs, dietary, and herbal supplement use.

Investigating a Modified Apparatus to Discriminate the Dissolution Capacity In Vitro and Establish an IVIVC of Mycophenolate Mofetil Tablets in the Fed State

In this study, a modified dissolution apparatus was developed by equipping a USP apparatus Ⅰ with an open-loop system to discriminate the dissolution capacity in vitro and establish an in vitro and in vivo correlation (IVIVC) for mycophenolate mofetil (MMF) tablets. MMF had strong pH-dependent solubility that could influence the dissolution rate in vivo after the meal. Dissolution tests involving reference (Cellcept®) and test formulations (F1 and F2) were conducted using pH 4.5 acetate buffer to simulate gastric fluids in the fed state. The dissolution profiles of the reference and test formulations were distinguished by using the modified dissolution apparatus and compared with those determined using the USP apparatuses Ⅱ and Ⅳ, and the dissolution capacities of the formulations were discriminated at different sampling time-points. The results of human bioequivalence (BE) studies in the fed state were consistent with in vitro evaluations that the maximum concentrations (C_max,in vivo) of both F1 and F2 fell below the acceptable range (80.00%). A level A IVIVC between the absorption fraction in vivo and dissolution in vitro, and a level C correlation between C_max,in vivo and C_{max,in vitro}, were established to guide the optimization of the tablet formulation containing MMF.

Generic immunosuppressants

Immunosuppressive drugs for solid organ transplantation are critical dose drugs with a narrow therapeutic index. Many of the most commonly used innovator drugs are off patent and have been replicated by generic counterparts, often at substantial cost-savings to the patient. However, serious adverse events caused by the transition from innovator to generic medications, specifically in pediatric solid organ transplant recipients, have questioned these autosubstitutions. The purpose of this review is to summarize the criteria set forth by the regulatory bodies, and to examine how major immunosuppressive drugs conform to these recommendations. Regulatory bodies have established inconsistent criteria to demonstrate bioequivalence between innovator and generic medications, causing approved generic variations to have varying levels of equivalence with the innovator drugs. In order to minimize the risk for under-immunosuppression, the following recommendations have been concluded. Brand prescribing of cyclosporine and tacrolimus are recommended due to evidence of adverse events after conversion to generic formulations and differences in dissolution parameters. Mycophenolate mofetil (MMF) shows better bioequivalence between innovator and generic formulations, however caution should be advised when switching between formulations. The institution of 'innovator only' policies may be appropriate at this time in order to minimize the risk of under-immunosuppressing patients until the evidence of more stringent bioequivalence has been established.

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.