James E. Polli

Biopharmaceutics classification system: the scientific basis for biowaiver extensions.

The current BSC guidance issued by the FDA allows for biowaivers based on conservative criteria. Possible new criteria and class boundaries are proposed for additional biowaivers based on the underlying physiology of the gastrointestinal tract. The proposed changes in new class boundaries for solubility and permeability are as follows: 1. Narrow the required solubility pH range from 1.0-7.5 to 1.0-6.8. 2. Reduce the high permeability requirement from 90% to 85%. The following new criterion and potential biowaiver extension require more research: 1. Define a new intermediate permeability class boundary. 2. Allow biowaivers for highly soluble and intermediately permeable drugs in IR solid oral dosage forms with no less than 85% dissolved in 15 min in all physiologically relevant dissolution media, provided these IR products contain only known excipients that do not affect the oral drug absorption. The following areas require more extensive research: 1. Increase the dose volume for solubility classification to 500 mL. 2. Include bile salt in the solubility measurement. 3. Use the intrinsic dissolution method for solubility classification. 4. Define an intermediate solubility class for BCS Class II drugs. 5. Include surfactants in in vitro dissolution testing.

Understanding Pharmaceutical Quality by Design

This review further clarifies the concept of pharmaceutical quality by design (QbD) and describes its objectives. QbD elements include the following: (1) a quality target product profile (QTPP) that identifies the critical quality attributes (CQAs) of the drug product; (2) product design and understanding including identification of critical material attributes (CMAs); (3) process design and understanding including identification of critical process parameters (CPPs), linking CMAs and CPPs to CQAs; (4) a control strategy that includes specifications for the drug substance(s), excipient(s), and drug product as well as controls for each step of the manufacturing process; and (5) process capability and continual improvement. QbD tools and studies include prior knowledge, risk assessment, mechanistic models, design of experiments (DoE) and data analysis, and process analytical technology (PAT). As the pharmaceutical industry moves toward the implementation of pharmaceutical QbD, a common terminology, understanding of concepts and expectations are necessary. This understanding will facilitate better communication between those involved in risk-based drug development and drug application review.

The Use of BDDCS in Classifying the Permeability of Marketed Drugs

We recommend that regulatory agencies add the extent of drug metabolism (i.e., ≥ 90% metabolized) as an alternate method in defining Class 1 marketed drugs suitable for a waiver of in vivo studies of bioequivalence. That is, ≥ 90% metabolized is an additional methodology that may be substituted for ≥ 90% absorbed. We propose that the following criteria be used to define ≥ 90% metabolized for marketed drugs: Following a single oral dose to humans, administered at the highest dose strength, mass balance of the Phase 1 oxidative and Phase 2 conjugative drug metabolites in the urine and feces, measured either as unlabeled, radioactive labeled or nonradioactive labeled substances, account for ≥ 90% of the drug dosed. This is the strictest definition for a waiver based on metabolism. For an orally administered drug to be ≥ 90% metabolized by Phase 1 oxidative and Phase 2 conjugative processes, it is obvious that the drug must be absorbed. This proposal, which strictly conforms to the present ≥ 90% criteria, is a suggested modification to facilitate a number of marketed drugs being appropriately assigned to Class 1.

Rapid Identification of P-glycoprotein Substrates and Inhibitors

Identifying molecules that interact with P-glycoprotein (P-gp) is important for drug discovery but is also generally reliant on time-consuming in vitro and in vivo studies. As an alternative approach, the current study applied pharmacophore models and database screening to rapidly retrieve molecules that bind as substrates or inhibitors for P-gp from commercial databases and then confirmed their affinity as inhibitors in vitro. Seven molecules (acitretin, cholecalciferol, misoprostol, nafcillin, repaglinide, salmeterol, and telmisartan) with no published details for P-gp affinity, one positive control inhibitor (miconazole), and two negative control molecules (phenelzine and zonisamide) were selected for testing. The MDCK-MDR1 in vitro cell model was used to confirm their inhibitory effect on [3H]digoxin transport, and the ATPase assay was used as an additional in vitro tool to indicate P-gp activation. All seven test drugs were confirmed to have P-gp affinity. Additionally, our experimental results provided plausible explanations for the published pharmacokinetic profiles of the tested drugs and their classification according to the biopharmaceutics and drug disposition classification system. In this study, we showed the successful application of pharmacophore models to accurately predict P-gp binding, which holds promise to anticipate drug-drug interactions from screening drug databases and a priori prediction of novel P-gp inhibitors or substrates.

Influence of Passive Permeability on Apparent P-glycoprotein Kinetics

AbstractPurpose. The objectives of this work were to evaluate the importance of moderate passive permeability on apparent P-glycoprotein (P-gp) kinetics, and demonstrate that inspection of basolateral to apical and apical to basolateral (BL-AP/AP-BL) permeability ratios may result in a compound being overlooked as a P-gp substrate and inhibitor of another drugs transport via P-gp inhibition. Methods. The permeability ratios of nicardipine, vinblastine, cimetidine, and ranitidine were determined across Caco-2 monolayers that express P-gp, in the presence and absence of the specific P-gp inhibitor, GF120918. In addition, the permeability ratio of vinblastine was studied after pretreatment of Caco-2 monolayers with nicardipine, ranitidine, or cimetidine. Similar studies were repeated with hMDR1-MDCK monolayers. Results. The permeability ratios for cimetidine and vinblastine were >2. The permeability ratios for nicardipine and ranitidine were close to unity, and were not affected by the addition of GF120918. Based solely on ratios, only compounds with moderate transcellular permeability (vinblastine and cimetidine) would be identified as P-gp substrates. Although the permeability ratios appeared to be unity for nicardipine and ranitidine, both compounds affected the permeability of vinblastine, and were identified as substrates and inhibitors of P-gp. Studies performed in hMDR1-MDCK cells confirmed these experimental results. Data were explained in the context of a kinetic model, where passive permeability and P-gp efflux contribute to overall drug transport. Conclusions. Moderate passive permeability was necessary for P-gp to reduce the AP-BL drug permeability. Inspection of the permeability ratio after directional transport studies did not effectively identify P-gp substrates that affected the P-gp kinetics of vinblastine. Because of the role of passive permeability, drug interaction studies with known P-gp substrates, rather than directional permeability studies, are needed to elucidate a more complete understanding of P-gp kinetics.

Biowaiver monographs for immediate release solid oral dosage forms: Diclofenac sodium and diclofenac potassium†

Literature data are reviewed regarding the scientific advisability of allowing a waiver of in vivo bioequivalence (BE) testing for the approval of immediate release (IR) solid oral dosage forms containing either diclofenac potassium and diclofenac sodium. Within the biopharmaceutics classification system (BCS), diclofenac potassium and diclofenac sodium are each BCS class II active pharmaceutical ingredients (APIs). However, a biowaiver can be recommended for IR drug products of each salt form, due to their therapeutic use, therapeutic index, pharmacokinetic properties, potential for excipient interactions, and performance in reported BE/bioavailability (BA) studies, provided: (a) test and comparator contain the same diclofenac salt; (b) the dosage form of the test and comparator is identical; (c) the test product contains only excipients present in diclofenac drug products approved in ICH or associated countries in the same dosage form, for instance as presented in this paper; (d) test drug product and comparator dissolve 85% in 30 min or less in 900 mL buffer pH 6.8, using the paddle apparatus at 75 rpm or the basket apparatus at 100 rpm; and (e) test product and comparator show dissolution profile similarity in pH 1.2, 4.5, and 6.8.

Development of a more rapid, reduced serum culture system for Caco-2 monolayers and application to the biopharmaceutics classification system

The objectives were: (1) to develop a more rapid, reduced serum culture system for Caco-2 monolayers, relative to the traditional 21-day, 10% fetal bovine serum (FBS) system; and (2) to determine the biopharmaceutical drug classification of an oral therapeutic agent using this new system. Caco-2 cells were grown in the six well format on polycarbonate filters, in medium containing 2% iron supplemented calf serum (sCS) and a combination of growth factors and hormones. After 4 days in culture, permeabilities of three marker compounds (metoprolol, mannitol, and taurocholate) across monolayers were determined, and compared to permeabilities from the traditional 21-day, 10% FBS system, using cells at similar passage number. Cell morphology, degree of cell differentiation, and the presence of two efflux pumps were assessed. The 2% sCS model was also used to classify the permeability of an oral therapeutic agent as high or low. No difference in permeability was observed for metoprolol transport (P = 0.38) between the two culture methods, and the values obtained were independent of passage number of the cells. Mannitol permeability was about 2-fold higher from the 2% sCS system, as compared to the 10% FBS system. Taurocholate permeability was low indicating the 2% sCS culture at 4 days lacked this particular active transporter capability. Electron micrographs of cells grown in the 2% sCS system at 4 days revealed the presence of microvilli and tight junctions. P-glycoprotein and an efflux pump for furosemide were functionally present. The 2% sCS system indicated the oral therapeutic agent as highly permeable, which agreed with the 10% FBS system. This new system provides a rapid, accurate, and economical option for passive permeability determination, and appears to be applicable to the proposed Biopharmaceutics Classification System (BCS).

Prediction of dissolution–absorption relationships from a dissolution/Caco-2 system

While the analysis of in vitro dissolution-in vivo absorption relationships from oral solid dosage forms provides biopharmaceutical insight and regulatory benefit, no well developed method exists to predict dissolution-absorption relationships a priori to human studies. The objective was to develop an integrated dissolution/Caco-2 system to predict dissolution-absorption relationships, and hence the contributions of dissolution and intestinal permeation to overall drug absorption for fast and slow formulations of piroxicam, metoprolol, and ranitidine. Dissolution studies were conducted on fast and slow dissolving immediate-release formulations of piroxicam, metoprolol tartrate, and ranitidine HCl. Dissolution samples were treated with concentrated buffers to render them suitable (i.e., isotonic and neutral pH) for Caco-2 monolayer permeation studies. The dissolution/Caco-2 system yielded a predicted dissolution-absorption relationship for each formulation which matched the observed relationship from clinical studies. The dissolution/Caco-2 systems prediction of dissolution or permeation rate-limited absorption also agreed with the clinical results. For example, the dissolution/Caco-2 system successfully predicted the slow piroxicam formulation to be dissolution rate-limited, and the fast piroxicam formulation to be permeation rate-limited. Moreover, the system predicted this change from dissolution rate-limited absorption for slow piroxicam to permeation rate-limited absorption for fast piroxicam, in spite of piroxicams high permeability and low solubility. The dissolution/Caco-2 system may prove to be a valuable tool in formulation development. Broader evaluation of such a system is warranted.

In vitro studies are sometimes better than conventional human pharmacokinetic in vivo studies in assessing bioequivalence of immediate-release solid oral dosage forms.

Human pharmacokinetic in vivo studies are often presumed to serve as the “gold standard” to assess product bioequivalence (BE) of immediate-release (IR) solid oral dosage forms. However, when this general assumption is re-visited, it appears that in vitro studies are sometimes better than in vivo studies in assessing BE of IR solid oral dosage forms. Reasons for in vitro studies to sometimes serve as the better method are that in vitro studies: (a) reduce costs, (b) more directly assess product performance, and (c) offer benefits in terms of ethical considerations. Reduced costs are achieved through avoiding in vivo studies where BE is self-evident, where biopharmaceutic data anticipates BE, and where in vivo BE study type II error is high. In vitro studies more directly assess product performance than do conventional human pharmacokinetic BE studies, since in vitro studies focus on comparative drug absorption from the two products, while in vivo BE testing can suffer from complications due to its indirect approach. Regarding ethical considerations, in vitro studies better embrace the principle “No unnecessary human testing should be performed” and can result in faster development. Situations when in vitro test should be viewed as preferred include Class I drugs with rapid dissolution, Class III drugs with very rapid dissolution, and highly variable drugs with rapid dissolution and that are not bio(equivalence)problem drugs. Sponsors of potential in vivo human pharmacokinetic BE testing should be required to justify why in vitro data is insufficient, similar to proposed animal testing requires justification to not employ an in vitro approach.

Transport of poly amidoamine dendrimers across Madin-Darby canine kidney cells

The objective of this study was to determine the permeability of a series of poly amidoamine (PAMAM) dendrimers of generations 0-4 (G0-G4) across MDCK (Madin-Darby Canine Kidney) cell line. PAMAM dendrimers with incremental increase in size and molecular weight were labeled by fluorescein isothiocyanate (FITC) and the least polydisperse fractions were collected by size exclusion chromatography. MDCK cells were grown on Transwell filters for four days. The conjugates were detected by HPLC equipped with fluorescence detector. The permeability of the dendrimers across MDCK cells was determined in the apical to basolateral direction. The rank-order permeability of the PAMAM dendrimers was G4 >> G1 approximately G0 > G3 > G2. The permeability of mannitol in the presence of G4 increased by nine-fold. Results suggest that the transepithelial transport of PAMAM dendrimers is effected by both the polymer size, and the modulation of the cell membrane by the cationic dendrimers.