In vitro ADME models make their mark in a Critical Path-driven environment.
The intent of the US Food and Drug Administration’s (FDA) Critical Path Initiative has been to modernize and accelerate the drug development process by making it less empirical, more scientific and rational. While much of the focus of the initiative has been on clinical trials, including validation of biomarkers and surrogate endpoints, many of us in the pharmaceutical industry have been making contributions on the preclinical end as well. The purpose of this article is to highlight the impact of some in vitro models on clinical trials, consistent with the spirit of the Critical Path Initiative.
In the early 1990s, about 40% of drug candidates failed in clinical trials due to poor Absorption, Distribution, Metabolism and Excretion (ADME) properties. Fifteen years later, that number is around 10%. Why? The main reason is that in vitro testing, some of which is now required by the FDA, identifies potential problems long before a compound reaches the clinic. Today, toxicity and poor efficacy result in most clinical trial failures. This is largely due to the fact that it remains difficult to predict clinical efficacy and toxicity based on preclinical models, either in vitro or in vivo. Again, this emphasizes the value of reliable, predictive in vitro model systems.
The development of in vitro ADME models, beginning with metabolism, can be traced back to the case of a single drug, terfenadine (Seldane), the first of the non-sedating antihistamines. By the late 1980s, it began to dawn on clinicians that an alarming number of cases of sudden death could be attributed to the drug (Monahan BP et al. Torsades de Pointes occurring in association with terfenadine use. JAMA 1990; 264(21):2788-2790). The cause, as we now know, was a harmful interaction with certain co-administered drugs, typically antibiotics or antifungals, which inhibit CYP3A4, the liver enzyme that breaks down terfenadine. When a patient who was taking Seldane started taking a CYP3A4 inhibitor such as erythromycin or ketoconazole, the concentration of one isomer of terfenadine rapidly and dramatically increased to such an extent that it caused an often-fatal cardiac arrhythmia called Torsades de Pointes. The point is that the mechanism of the drug-drug interaction was discovered through the use of an in vitro model, namely human liver microsomes, that was not readily available prior to that time and was certainly not used routinely to screen new drug candidates.
By 1997, when the FDA mandated the use of human in vitro models to test for potential metabolism-based drug interactions, the pharmaceutical industry was already on board. A 1999 FDA guidance on in vivo drug metabolism and drug interaction studies, as well as an update that was published in draft form in 2006, specified the clinical consequences of in vitro drug interaction findings: a positive result requires a follow-up clinical drug interaction trial, but a negative result in vitro is sufficiently definitive on its own.
But enough about drug metabolism! Absorption can also be predicted in vitro. Caco-2 cells were first characterized as an in vitro model for predicting intestinal drug absorption by Hidalgo, Raub and Borchardt in 1989 and eventually became the de facto standard for that purpose in the pharmaceutical industry. The correlation between apparent permeability across a Caco-2 monolayer in vitro and absorption of orally-administered drugs in vivo is well established and responsible, in part, for the aforementioned decrease in the number of clinical drug failures attributable to poor ADME properties. Sponsors of NCEs can now select, for clinical development, those drug candidates with desirable ADME properties, based on in vitro testing.
One aspect of the distribution and excretion of drugs is their interaction with membrane proteins called transporters. Uptake transporters are required for the uptake of some drugs into cells, whereas efflux transporters are responsible for pumping some drugs out of cells or preventing them from ever getting in. Interactions with transporters (or lack thereof) can account for many differences between drugs in terms of systemic bioavailability (via intestinal absorption or uptake and subsequent metabolism in the liver), side effects (e.g., via efflux transporters in the blood-brain barrier), toxicity (e.g., via placental efflux transporters), efficacy (e.g., via efflux transporters expressed in cancer cells), and biliary or renal excretion (via uptake and efflux transporters in the liver and kidney, respectively). Transporters can also be involved in drug-drug interactions, a fact recognized by the FDA in its 2006 draft guidance on drug interaction studies. Through the use of a variety of in vitro models, we now understand the basis for a number of clinical drug interactions (e.g., the increase in bioavailability of digoxin, a P-glycoprotein substrate, seen with co-administered quinidine, an inhibitor of the same transporter). Increasingly, we can now predict, at least qualitatively, such interactions. The FDA now expects, and will soon require, in vitro testing for interactions with drug transporters. The results of such testing will inform the design of clinical trials in the same way as in vitro testing for metabolism-based drug-drug interactions.
In vitro models such as Caco-2 have had an equally dramatic impact on the development of generic drugs. The 2000 FDA guidance on the Biopharmaceutics Classification System, or BCS, established a mechanism by which a generic drug developer could obtain a “biowaiver” based on in vitro data. A biowaiver means that a clinical bioequivalence study need not be carried out—another example where appropriate in vitro data, obtained with a properly validated system, can substitute for an otherwise necessary clinical trial, thereby saving several months and hundreds of thousands of dollars. A BCS biowaiver can also apply to new drug development, where an average of four to six clinical bioequivalence studies are performed (e.g., on different formulations or manufacturing processes) during the development of a typical new chemical entity (NCE). It has taken the industry a while to catch on to the significance of the BCS: Absorption Systems performed over 50 such studies between 2001 and 2007, over half of them in 2007 alone.
Most people on the development (as opposed to discovery) side of the pharmaceutical fence probably give little or no thought to in vitro studies. But they should. The FDA allows sponsors to avoid certain clinical trials based on appropriate in vitro data, so why not take advantage of that?
About the Author
Prior to joining Absorption Systems, Bode was Vice President of Operations for Tissue Transformation Technologies, a leading provider of tissue-based reagents used in ADME studies.
Filed Under: Drug Discovery