The Biopharmaceutics Classification System (BCS) has found widespread utility in drug discovery, product development and drug product regulatory sciences. The classification scheme captures the two most significant factors influencing oral drug absorption; solubility and intestinal permeability and it has proven to be a very useful and a widely accepted starting point for drug product development and drug product regulation. The mechanistic base of the BCS approach has, no doubt, contributed to its wide spread acceptance and utility. Nevertheless, underneath the simplicity of BCS are many detailed complexities, both in vitro and in vivo which must be evaluated and investigated for any given drug and drug product. In this manuscript we propose a simple extension of the BCS classes to include subspecification of acid (a), base (b) and neutral (c) for classes II and IV. Sub-classification for Classes I and III (high solubility drugs as currently defined) is generally not needed except perhaps in border line solubility cases. It is well known that the , pKa physical property of a drug (API) has a significant impact on the aqueous solubility dissolution of drug from the drug product both in vitro and in vivo for BCS Class II and IV acids and bases, and is the basis, we propose for a sub-classification extension of the original BCS classification. This BCS sub-classification is particularly important for in vivo predictive dissolution methodology development due to the complex and variable in vivo environment in the gastrointestinal tract, with its changing pH, buffer capacity, luminal volume, surfactant luminal conditions, permeability profile along the gastrointestinal tract and variable transit and fasted and fed states. We believe this sub-classification is a step toward developing a more science-based mechanistic in vivo predictive dissolution (IPD) methodology. Such a dissolution methodology can be used by development scientists to assess the likelihood of a formulation and dosage form functioning as desired in humans, can be optimized along with parallel human pharmacokinetic studies to set a dissolution methodology for Quality by Design (QbD) and in vitro–in vivo correlations (IVIVC) and ultimately can be used as a basis for a dissolution standard that will ensure continued in vivo product performance.

The FDA has published recommendations for sponsors who wish to request a waiver of in vivo bioavailability (BA) or bioequivalence (BE) studies for immediate release (IR) solid oral dosage forms based on the Biopharmaceutics Classification System (BCS). Biowaivers can be requested for IR formulations in which the active ingredient is shown to be a BCS class I drug: that is, a drug showing high permeability and high solubility over a pH range of 1-7.5. For permeability determinations, a variety of experimental methods can be used, such as the rat in situ single pass perfusion or Caco-2 cell culture models, once the suitability of the particular method is established. Following the recommended procedure for assessing the suitability of permeability determinations, we determined the permeability of 20 test drugs using the in situ single pass perfusion model in rats. The test compounds were coperfused through jejunal intestinal segments with an internal permeability reference standard (metoprolol) over a 90 min time period. Sample analysis was performed by HPLC, and the ratio of the effective permeability, Peff (cm/s), of test compound to that of metoprolol was determined. To address the question of test drug permeabilities that approach that of the internal standard, we propose that a statistical analysis such as the "0.8-1.25 rule" used for in vivo or in vitro bioequivalence studies provide guidance for permeability classification using the in situ single pass perfusion model. We developed a method using the 90% confidence interval of the permeability ratio of the test to internal reference standard in order to differentiate between high and low permeability compounds. This analysis allowed for the proper permeability classification of all of the test compounds and suggests a robust means for assessing drug permeability classification.

In this study, we determined the pH and buffer capacity of human gastrointestinal (GI) fluids (aspirated from the stomach, duodenum, proximal jejunum, and mid/distal jejunum) as a function of time, from 37 healthy subjects after oral administration of an 800 mg immediate-release tablet of ibuprofen (reference listed drug; RLD) under typical prescribed bioequivalence (BE) study protocol conditions in both fasted and fed states (simulated by ingestion of a liquid meal). Simultaneously, motility was continuously monitored using water-perfused manometry. The time to appearance of phase III contractions (i.e., housekeeper wave) was monitored following administration of the ibuprofen tablet. Our results clearly demonstrated the dynamic change in pH as a function of time and, most significantly, the extremely low buffer capacity along the GI tract. The buffer capacity on average was 2.26 μmol/mL/ΔpH in fasted state (range: 0.26 and 6.32 μmol/mL/ΔpH) and 2.66 μmol/mL/ΔpH in fed state (range: 0.78 and 5.98 μmol/mL/ΔpH) throughout the entire upper GI tract (stomach, duodenum, and proximal and mid/distal jejunum). The implication of this very low buffer capacity of the human GI tract is profound for the oral delivery of both acidic and basic active pharmaceutical ingredients (APIs). An in vivo predictive dissolution method would require not only a bicarbonate buffer but also, more significantly, a low buffer capacity of dissolution media to reflect in vivo dissolution conditions.

Dipeptide monoester prodrugs of floxuridine were synthesized, and their chemical stability in buffers, resistance to glycosidic bond metabolism, affinity for PEPT1, enzymatic activation and permeability in cancer cells were determined and compared to those of mono amino acid monoester floxuridine prodrugs. Prodrugs containing glycyl moieties were the least stable in pH 7.4 buffer (t1/2 < 100 min). The activation of all floxuridine prodrugs was 2- to 30-fold faster in cell homogenates than their hydrolysis in buffer, suggesting enzymatic action. The enzymatic activation of dipeptide monoester prodrugs containing aromatic promoieties in cell homogenates was 5- to 20-fold slower than that of other dipeptide and most mono amino acid monoester prodrugs (t1/2 ∼ 40 to 100 min). All prodrugs exhibited enhanced resistance to glycosidic bond metabolism by thymidine phosphorylase compared to parent floxuridine. In general, the 5′-O-dipeptide monoester floxuridine prodrugs exhibited higher affinity for PEPT1 than the corresponding 5′-O-mono amino acid ester prodrugs. The permeability of dipeptide monoester prodrugs across Caco-2 and Capan-2 monolayers was 2- to 4-fold higher than the corresponding mono amino acid ester prodrug. Cell proliferation assays in AsPC-1 and Capan-2 pancreatic ductal cell lines indicated that the dipeptide monoester prodrugs were equally as potent as mono amino acid prodrugs. The transport and enzymatic profiles of 5′-l-phenylalanyl-l-tyrosyl-floxuridine, 5′-l-phenylalanyl-l-glycyl-floxuridine, and 5′-l-isoleucyl-l-glycyl-floxuridine suggest their potential for increased oral uptake, delayed enzymatic bioconversion and enhanced resistance to metabolism to 5-fluorouracil, as well as enhanced uptake and cytotoxic activity in cancer cells, attributes that would facilitate prolonged systemic circulation for enhanced therapeutic action.