The 2013 GPhA/FDA Fall Technical Conference placed great emphasis on quality and compliance issues, and how GDUFA addresses these.
In one of my favorite talks, Robert Lionberger (FDA OGD) addressed the growing complexity of generic products and the regulatory science initiatives surrounding such. Complex drugs may include those with complex APIs (e.g. peptides, natural source products), complex formulations (e.g. liposomes), complex routes of delivery (e.g. locally acting), or drug-device combinations (e.g. DPI, MDI, nasal sprays, and transdermal systems) - all controversial topics which have prompted citizen’s petitions.
This year featured a number of unprecedented bioequivalence (BE) guidances for complex products, including the first for an MDI (albuterol sulfate), an ophthalmic emulsion (cyclosporine), and a DPI (fluticasone proprionate/salmeterol xinafoate).
The most interesting, in my opinion, was the FDA’s stance on acyclovir ointment. The BE recommendation for this drug, published in March 2012, permits an in vitro option in lieu of a clinical endpoint study, assuming the generic formulation and RLD (Zovirax) are quantitatively and qualitatively the same (Q1/Q2), the formulations have similar physiochemical properties (viscosity, rheology, etc.), and comparable in vitro release rates.
This approach was not well-received by the innovator (Valeant) who, in their citizen’s petition to the FDA, claimed that permitting approval based on in vitro data for a locally-acting semisolid is “unprecedented and scientifically unsupportable”. Valeant claimed that such products are multiphasic with complex thermodynamics which could modify the release characteristics of the drug. Prior to the acyclovir BE recommendation, the only exceptions to a clinical endpoint study for locally-acting semisolids were 1) the Stoughton-McKenzie vasoconstriction or ‘skin blanching’ assay for corticosteroids and 2) in vivo microdialysis.
The FDA’s response to Valeant stated that there are two key concerns when determining BE for topical dermal products: 1) Are the test and reference formulated similarly such that release characteristics are the same between the two products, and 2) Will the amount of drug uptake by the skin be the same or will absorption be affected by differences in formulation and/or manufacturing of the two products? Furthermore, Zovirax is a unique product whose characteristics distinguish it from other topical semisolids: 1) The product does not have a multiphasic vehicle, rather it is a single API in a single ingredient vehicle (PEG), 2) the physiochemical characteristics which have potential to impact bioavailability are well-established, and 3) clinical endpoints are difficult given the modest clinical benefit shown for acyclovir.
This “characterization-based equivalence” for formulations with the same concentrations of the same inactive ingredients signifies the trend towards a rational, science-driven approach, relying on trusted in vitro methods (not unlike the notion of BCS biowaivers for solid oral dosage forms). Absorption Systems offers in vitro release tests for topical dermatological products and other semisolid dosage forms.
Lionberger also expressed the need for additional draft guidance on FDA recommendations for complex generics. For submission of complex products, the FDA recommends scheduling a pre-ANDA meeting.
Another interesting session was Johnny’s Young’s talk on ANDA filings and refuse to receive (RTR) issues, underscoring key points from the draft RTR guidance which was mandated by GDUFA. One audience member asked if they would be subject to RTR if they submit a BCS waiver application and the biowaiver option is not specifically listed in the product’s BE recommendation. The panel confirmed our experience that this would not be cause for an RTR. Young also advised applicants to consult the ANDA filing checklist which is updated quarterly (typically March, June, September, and December).
Dr. Susan Rosencrance (FDA OGD) provided a review of the Stability Guidance for ANDAs (finalized on June 20, 2013). This Guidance requests applicants to follow the stability recommendations provided in ICH Q1A-E guidelines, including 1) data from three pilot scale batches, or two pilot batches and one small scale batch, 2) six months accelerated and long-term data, 3) multiple lots of drug substance, 4) principles that are representative of the commercial process, 5) fully packaged primary exhibit batch, 6) three batches when using bracketing and matrixing designs, and 7) statistical analysis of the data as appropriate.
Rosencrance stated that the primary purpose of the Stability Guidance is to provide clarity in the stability expectations and a formal process for generic drugs that aligns with ICH. This is to promote harmonization between new and generic drugs, and an overall enhancement in generic drug quality. On the other hand, industry voiced concerns that that the new stability requirements represent significant changes and the time/resources required to fully implement. It is projected that the workload in stability labs for the enhanced stability testing will increase by 45% over three years and stability study budgets will increase by 40% over five years.
As the conference highlighted, the generic industry is facing new challenges and opportunities in introducing quality products. Whether in the design of complex products, in new stability provisions, or submission of applications – quality
AALAS (American Association for Laboratory Animal Science) held its 2013 national meeting recently in Baltimore. A primary focus of this meeting is technical advances in animal models, including tutorials on technically challenging manipulations and validation of a novel way of simulating a disease condition. I highlight one of each here.
Shaolan Li and Jennifer Scola of Vertex Pharmaceuticals presented a poster demonstrating a technique for sampling cerebrospinal fluid (CSF) via cisterna magna puncture in mice. The key is to obtain a sufficient volume of CSF (5 to 10 μL in this case), free of contamination by blood, to enable quantification of the amount of a test compound that has crossed the blood-brain barrier. The CSF:plasma concentration ratio is an important parameter, both for compounds targeting the central nervous system (CNS) and for non-CNS compounds. With this technique, it is possible to obtain both CSF and plasma concentrations in the same animal, which is generally quite challenging in mice. CSF sampling is performed with anesthetized animals in a stereotaxic apparatus. Key considerations include proper fixation of the head and positioning of the body, gentle dissection of overlying tissues, avoiding blood vessels (which are clearly visible) when penetrating the dura mater at a shallow angle, and limiting the amount of CSF withdrawn and the rate of withdrawal. Although the technique described is currently a terminal procedure, the authors anticipate that it may be possible to perform it as a survival procedure in the future. We are already practicing the technique and look forward to incorporating it into relevant pharmacokinetic study designs soon.
Glen Gum and colleagues from Absorption Systems took third place in the poster competition with their presentation on the development of a novel, economical prototype chamber for inducing dry eye (keratoconjunctivitis sicca, or KCS) in rodents. The novel design, which incorporates two small fans in a standard rodent enclosure, is an alternative to more expensive controlled environmental chambers. Computer-aided design (CAD) was used to simulate and optimize the airflow in the chamber, helping to direct design modifications. Measured airflow in the chamber was stable and consistent, and it produced the desired results: within three days, tear production in mice was less than 50% of control and the corneal epithelial layer was compromised. This is a model that we can now use to test treatments for dry eye.
At the AALAS meeting and in every field in which Absorption Systems is active, we remain on the cutting edge and actively involved in advancing the science.
It’s important to assess potential drug disposition in the central nervous system (CNS) to understand desired or undesired pharmacological effects. One of the most commonly used tools for screening penetration of the blood-brain-barrier (BBB) is the immortalized cell line, MDR1-MDCK. These cells, which form polarized monolayers, overexpress human P-gp (MDR1), the most well-characterized efflux transporter on the luminal side of the BBB. Most MDR1-MDCK cells lines are derived from one of two origins: 1) the National Institutes of Health (NIH) or 2) the Netherlands Cancer Institute (NKI; Borst cell line).
Kikuchi et al., recently conducted an extensive literature review to compare the P-gp efflux ratio (ER) obtained in vitro (from the MDR1-MDCK cell lines) to in vivo ER. They found a strong correlation between in vivo and in vitro ER, even though the data were obtained from various literature sources from multiple labs. Of note, the ER of the NIH MDR1-MDCK cell line was found to be a better predictor of in vivo ER compared to the NKI MDR1-MDCK cell line, with r2 values of 0.813 vs. 0.531, respectively (Kikuchi et al, 2013).
Absorption Systems has worked with and characterized the NIH MDR1-MDCK cell line for more than a decade. It is a key component of our integrated BBB package. In fact, much of the data referenced in the Kikuchi publication (Broaccatelli et al) was sourced from Absorption Systems original data (Summerfield et al)
As the field of drug transporters continues to evolve and additional transporters are deemed ‘clinically relevant’ with regard to drug-drug interactions, drug innovators are faced with selecting in vitro test systems that can not only demonstrate transporter functionality but also generate data that is more quantitative as well as translatable to human outcomes. This is a far cry from where the field was in 2006, when transporters first officially appeared on the FDA’s radar screen in the form of a draft guidance. At the recent DMDG open meeting in Cambridge, UK , there were several subjects at the forefront of the discussion; one of them being transporters and the how to approach in vitro studies. Dr. Agnes Poirier of Hoffman-La Roche AG in Basel presented on the application of FDA-defined thresholds for transporter inhibition assessment. Using a training set of compounds, she and her colleagues applied the FDA’s [I]/IC50 thresholds [I]1/IC50 ≥0.1 and [I]2/IC50 ≥10 (where [I]1 is defined as the mean steady-state total Cmax and [I]2 is an estimate of the gut concentration, equal to dose/250 mL) to determine the accuracy of these cutoffs. These values have been defined based on clinical data and results from a variety of in vitro assays (based on different cell-based and vesicle-based test systems). Based on the assessment by Dr. Poirier, et al. (due for publication in the fall of 2013), [I]2/IC50 resulted in fewer false negatives when using the FDA-defined cutoff but still had some false negatives. However, Roche did not stop there; they moved forward to define their own internal thresholds based on clinical data and results from their in vitro test systems. Ultimately, through the calibration of their test systems to the clinical data, they have defined thresholds that are accurate for their test systems and most translatable to clinical outcomes and will apply these values moving forward. I look forward to this publication very much.
The data presented by Dr. Poirer got me thinking; often, we are more concerned with the outcome of a study involving an in vitro test system (e.g., is my compound an inhibitor or a substrate?) than the characterization and calibration of the test system itself. True translatability of a test system begins with the choice and definition of parameters for use of the test system, before moving forward with assessment of novel compounds. And this is not only limited to the in vitro assessment of a transporter interaction, but also to in silico predictions. Developing in silico models for transporter interactions, highlighted at the DMDG meeting by Dr. Rob Elsby of AstraZeneca, also relies heavily on access to good-quality in vitro data. A model is only as good as the data used to build it – this concept and the different types of models available were recently highlighted in a publication by the International Transporter Consortium (ITC). In additional to this publication, I would also recommend a recent AAPS webinar entitled “The Role of Transporters on Unbound Intracellular Concentrations: Experimental and Modeling Considerations.” These are great resources for the basics on available models and modeling.
As the drug transporter field evolves, so do the available tools. While there are a number of systems available (in vitro, in vivo, and in silico), the road to translatability always begins with thorough characterization and calibration. Although the importance of this step is frequently underestimated (but not by Absorption Systems), confidence in the test system leads to confidence in actionable conclusions for assessment of transporter DDI risks.
The theme of this symposium was that, from an ADME perspective, children are not just small adults and should not be approached as such with regard to drug development.
In the course of describing pre-clinical juvenile toxicology studies to support pediatric clinical trials, Jennifer Ingram-Ross of J&J mentioned that acetaminophen is less hepatotoxic to children than adults. That’s probably a good thing, since there are so many OTC products for children and it would be easy to overdose them with acetaminophen. But it made me wonder why it is less toxic to children: do they express lower levels of CYP2E1 (perhaps as a consequence of consuming no alcohol)? Do they have higher levels of GSH? Not sure, but while discussing the question with Steve Leeder (Children’s Mercy Hospital, Kansas City, MO) he turned me on to some work that I was previously unaware of on a circulating protein adduct that appears to be a biomarker of acetaminophen hepatotoxicity.
Lauren Aleksunes (Rutgers) is studying the global down-regulation of maternal drug transporters during pregnancy and lactation, which may be responsible for intracellular cholestasis of pregnancy (2% prevalence in humans). In vitro, at least, placental lactogen down-regulates uptake and efflux transporters in primary human hepatocytes. I found it particularly interesting that not only the fetal liver, but also the mother’s liver, expresses CYP3A7 during this time.
Steve Leeder discussed the ontogeny of CYPs in humans, particularly enzymes with a polymorphic phenotype determined by genetic variation. For CYP2D6, genetic variation is more important than ontogeny at all ages, since its expression is turned on (except in poor metabolizers) by two weeks of age. Not so for CYP2C19, where ontogeny is more important early and genetic variation more important later. He was followed by Amin Rostami-Hodjegan (Simcyp, University of Manchester, UK), who drove home the point that drug-drug interaction risk may be very different in young children vs. adults due to the importance of the fraction of a victim drug metabolized by a given CYP. In an adult, a drug that is metabolized equally by two different CYPs, inhibition of one or the other will have little effect on exposure. But if the expression of one of those enzymes is absent or very low at birth and increases gradually during development, inhibition of the other enzyme in an infant could have dire consequences.
In discussing the pharmacogenetics of mycophenolic acid (MPA), Tristan Sissung (NCI) made one observation that, to me, pointed out the complexity of drug transporters and the difficulty of predicting what will happen when a transporter is blockd. Inhibition of MRP2 (for which MPA is a substrate), for example by cyclosporine A (which is frequently co-administered with MPA following an organ transplant), decreases the AUC of MPA by 30% due to interference with enterohepatic recirculation. In general, I would expect to see an increase in exposure when an efflux transporter such as MRP2 is blocked because of an increase in absorption and slower elimination.
Rest assured that Absorption Systems remains on the cutting edge of ADME research and is ever ready to characterize your new drug candidates. Check out our recent peer-reviewed publications here and here.