The use of biopharmaceutic classification of drugs in drug discovery and development: current status and future extension

1-Alkyl-4-phenyl-6-alkoxy-1H-quinazolin-2-ones: a novel series of potent calcium-sensing receptor antagonists

Parathyroid hormone (PTH) is an effective bone anabolic agent. However, only when administered by daily sc injections exposure of short duration is achieved, a prerequisite for an anabolic response. Instead of applying exogenous PTH, mobilization of endogenous stores of the hormone can be envisaged. The secretion of PTH stored in the parathyroid glands is mediated by a calcium sensing receptor (CaSR) a GPCR localized at the cell surface. Antagonists of CaSR (calcilytics) mimic a state of hypocalcaemia and stimulate PTH release to the bloodstream. Screening of the internal compound collection for inhibition of CaSR signaling function afforded 2a. In vitro potency could be improved >1000 fold by optimization of its chemical structure. The binding mode of our compounds was predicted based on molecular modeling and confirmed by testing with mutated receptors. While the compounds readily induced PTH release after iv application a special formulation was needed for oral activity. The required profile was achieved by using microemulsions. Excellent PK/PD correlation was found in rats and dogs. High levels of PTH were reached in plasma within minutes which reverted to baseline in about 1-2 h in both species.

Evaluation and suggested improvements of the Biopharmaceutics Classification System (BCS)

This review has evaluated the Biopharmaceutics Classification System (BCS) and improvements have been proposed. The BCS has a very strict solubility/dissolution limit, a generous Pe-limit (≥ 14-times higher rate constant limit for dissolution than for permeation), and is stricter for drugs with a long half-life (t1/2). Available human in-vivo, in-vitro, and in-silico Pe-methods cannot classify Pe for moderately to highly permeable substances sufficiently well, and in-vitro data often underpredict the in-vivo dissolution potential and rate. Good in-vivo dissolution and absorption can be expected for most high Pe drug products. It has not been possible to find a highly permeable product with a Dose number (Do) < 385 (< 2400 in the fed state) that is clearly incompletely absorbed, and near complete uptake has been shown for a drug product with a Do of 660000. The potential implication of these findings is that many true BCS Class I drug products are incorrectly classified. This could be a reason for the limited use of this system. On this basis, it has been suggested that: the limit for high for solubility/dissolution is decreased (to >40 and >95% dissolved within 30min and 3 h, respectively); the limit for high Pe is increased (to >Pe of metoprolol); accurate Pe-models or in-vivo fraction absorbed data are used; solubility/dissolution tests are performed using real or validated simulated gastrointestinal fluids; in-vitro/in-vivo dissolution relationships are established; the t½ is considered; and the rate-limiting step for in-vivo absorption is determined. A major change could be to reduce the BCS into two classes: permeation-rate (Class I) or dissolution-rate (Class II) limited absorption. It is believed that this could give a better balance and increase the number of biowaivers.

Prediction of solubility and permeability class membership: provisional BCS classification of the world's top oral drugs

The Biopharmaceutics Classification System (BCS) categorizes drugs into one of four biopharmaceutical classes according to their water solubility and membrane permeability characteristics and broadly allows the prediction of the rate-limiting step in the intestinal absorption process following oral administration. Since its introduction in 1995, the BCS has generated remarkable impact on the global pharmaceutical sciences arena, in drug discovery, development, and regulation, and extensive validation/discussion/extension of the BCS is continuously published in the literature. The BCS has been effectively implanted by drug regulatory agencies around the world in setting bioavailability/bioequivalence standards for immediate-release (IR) oral drug product approval. In this review, we describe the BCS scientific framework and impact on regulatory practice of oral drug products and review the provisional BCS classification of the top drugs on the global market. The Biopharmaceutical Drug Disposition Classification System and its association with the BCS are discussed as well. One notable finding of the provisional BCS classification is that the clinical performance of the majority of approved IR oral drug products essential for human health can be assured with an in vitro dissolution test, rather than empirical in vivo human studies.

Colonic treatments and targets: issues and opportunities

The colon provides a plethora of therapeutic opportunities. There are multiple disease targets, drug molecules, and colon-specific delivery systems to be explored. Clinical studies highlight the potential for systemic delivery via the colon, and the emerging data on the levels of cell membrane transporters and metabolic enzymes along the gut could prove advantageous for this. Often efflux transporters and metabolic enzyme levels are lower in the colon, suggesting a potential for improved bioavailability of drug substrates at this site. The locoregional distribution of multiple metabolic enzymes (including cytochromes), efflux transporters (including P-glycoprotein and breast cancer resistance proteins), and influx transporters (including the solute carrier family) along the intestine is summarized. Local delivery to the colonic mucosa remains a valuable therapeutic option. New therapies that target inflammatory mediators could improve the treatment of inflammatory bowel disease, and old and new anticancer molecules could, when delivered topically, prove to be beneficial adjuncts to the current systemic or surgical treatments. New issues such as pharmacogenomics, chronotherapeutics, and the delivery of prebiotics and probiotics are also discussed in this review. Targeting drugs to the colon utilizes various strategies, each with their advantages and flaws. The most promising systems are considered in the light of the physiological data which influence their in vivo behavior.

Pharmaceutical applications of AC biosusceptometry

AC Biosusceptometry offers an alternative to investigate noninvasively and without ionizing radiation the behavior of solid dosage forms in vitro and in the human gastrointestinal tract. This versatility allowed applying this technique in a wide field ranging from characterization of the disintegration process to elucidation of how the physiological parameters can interfere with pharmaceutical processes. It is increasingly important to understand how oral solid dosage forms behave in the human gastrointestinal tract. Once labelled, magnetic dosage forms provide an excellent opportunity to investigate complexes' interactions between dosage form and gastrointestinal physiology. In this paper, basic principles of this biomagnetic instrumentation and of the quantification based on magnetic images are reviewed. Also will be presented are some of the most recent applications of AC Biosusceptometry in the pharmaceutical research including oesophageal transit, gastric emptying and transit time of multiparticulate dosage forms, hydrophilic matrices and disintegration of tablets.

The use of drug metabolism for prediction of intestinal permeability (dagger)

The Biopharmaceutics Classification System (BCS), based on the aqueous solubility and intestinal permeability of a drug substance, has been widely used to predict the extent of drug absorption during the course of pharmaceutical development. Combined with product dissolution data, this system has gained a prominent role in regulatory process to determine if a drug formulated in an immediate release solid oral dosage form qualifies for waiver of in vivo bioequivalence studies. In parallel, the Biopharmaceutics Drug Disposition Classification System (BDDCS), using aqueous solubility and drug metabolism, takes on another venue to predict overall drug disposition. It has been suggested that the matrix of drug metabolism in BDDCS can be used to substantiate the classification of permeability by BCS. A total of 51 drugs were compiled in this study to examine the use of drug metabolism for predicting permeability. All compounds were classified as high permeability based on BCS, but only 73% of the compounds were found to exhibit extensive metabolism. Lipophilicity accounts for significant metabolism of many highly permeable drugs. Fourteen (14) out of 51 drugs have poor metabolism, suggesting that high permeability as defined by BCS does not necessarily dictate extensive metabolism. The drugs that have high permeability but poor metabolism are generally hydrophilic molecules with low molecular weight and are likely to be absorbed by active transport mechanisms. Based on the present data and literature information, it seems logical to predict that the extent of absorption is mostly complete (or > or =90%) if the drug is subject to a high degree of metabolism (e.g., > or =90%). The extent of drug metabolism may be useful in supporting permeability classification under certain circumstances.

Iron behaving badly: inappropriate iron chelation as a major contributor to the aetiology of vascular and other progressive inflammatory and degenerative diseases

BACKGROUND

The production of peroxide and superoxide is an inevitable consequence of aerobic metabolism, and while these particular 'reactive oxygen species' (ROSs) can exhibit a number of biological effects, they are not of themselves excessively reactive and thus they are not especially damaging at physiological concentrations. However, their reactions with poorly liganded iron species can lead to the catalytic production of the very reactive and dangerous hydroxyl radical, which is exceptionally damaging, and a major cause of chronic inflammation.

REVIEW

We review the considerable and wide-ranging evidence for the involvement of this combination of (su)peroxide and poorly liganded iron in a large number of physiological and indeed pathological processes and inflammatory disorders, especially those involving the progressive degradation of cellular and organismal performance. These diseases share a great many similarities and thus might be considered to have a common cause (i.e. iron-catalysed free radical and especially hydroxyl radical generation).The studies reviewed include those focused on a series of cardiovascular, metabolic and neurological diseases, where iron can be found at the sites of plaques and lesions, as well as studies showing the significance of iron to aging and longevity. The effective chelation of iron by natural or synthetic ligands is thus of major physiological (and potentially therapeutic) importance. As systems properties, we need to recognise that physiological observables have multiple molecular causes, and studying them in isolation leads to inconsistent patterns of apparent causality when it is the simultaneous combination of multiple factors that is responsible.This explains, for instance, the decidedly mixed effects of antioxidants that have been observed, since in some circumstances (especially the presence of poorly liganded iron) molecules that are nominally antioxidants can actually act as pro-oxidants. The reduction of redox stress thus requires suitable levels of both antioxidants and effective iron chelators. Some polyphenolic antioxidants may serve both roles.Understanding the exact speciation and liganding of iron in all its states is thus crucial to separating its various pro- and anti-inflammatory activities. Redox stress, innate immunity and pro- (and some anti-)inflammatory cytokines are linked in particular via signalling pathways involving NF-kappaB and p38, with the oxidative roles of iron here seemingly involved upstream of the IkappaB kinase (IKK) reaction. In a number of cases it is possible to identify mechanisms by which ROSs and poorly liganded iron act synergistically and autocatalytically, leading to 'runaway' reactions that are hard to control unless one tackles multiple sites of action simultaneously. Some molecules such as statins and erythropoietin, not traditionally associated with anti-inflammatory activity, do indeed have 'pleiotropic' anti-inflammatory effects that may be of benefit here.

CONCLUSION

Overall we argue, by synthesising a widely dispersed literature, that the role of poorly liganded iron has been rather underappreciated in the past, and that in combination with peroxide and superoxide its activity underpins the behaviour of a great many physiological processes that degrade over time. Understanding these requires an integrative, systems-level approach that may lead to novel therapeutic targets.

Intestinal permeability and its relevance for absorption and elimination

Human jejunal permeability (P(eff)) is determined in the intestinal region with the highest expression of carrier proteins and largest surface area. Intestinal P(eff) are often based on multiple parallel transport processes. Site-specific jejunal P(eff) cannot reflect the permeability along the intestinal tract, but they are useful for approximating the fraction oral dose absorbed. It seems like drugs with a jejunal P(eff) > 1.5 x 10(-4) cm s(-1) will be completely absorbed no matter which transport mechanism(s) are utilized. Many drugs that are significantly effluxed in vitro have a rapid and complete intestinal absorption (i.e. >85%) mediated by passive transcellular diffusion. The determined jejunal P(eff) for drugs transported mainly by absorptive carriers (such as peptide and amino acid transporters) will accurately predict the fraction of the dose absorbed as a consequence of the regional expression. The data also show that: (1) the human intestinal epithelium has a large resistance towards large and hydrophilic compounds; and (2) the paracellular route has a low contribution for compounds larger than approximately molecular weight 200. There is a need for more exploratory in vivo studies to clarify drug absorption and first-pass extraction along the intestine. One is encouraged to develop in vivo perfusion techniques for more distal parts of the gastrointestinal tract in humans. This would stimulate the development of more relevant and complex in vitro absorption models and form the basis for an accurate physiologically based pharmacokinetic modelling of oral drug absorption.

Prediction of human pharmacokinetics--gastrointestinal absorption

Permeability (P(e)) and solubility/dissolution are two major determinants of gastrointestinal (GI) drug absorption. Good prediction of these is crucial for predicting doses, exposures and potential interactions, and for selecting appropriate candidate drugs. The main objective was to evaluate screening methods for prediction of GI P(e), solubility/dissolution and fraction absorbed (f(a)) in humans. The most accurate P(e) models for prediction of f(a) of passively transported and highly soluble compounds appear to be the 2/4/A1 rat small intestinal cell model (in-vitro and in-silico), a newly developed artificial-membrane method, and a semi-empirical approach based on in-vitro membrane affinity to immobilized lipid bilayers, effective molecular weight and physiological GI variables. The predictability of in-vitro Caco-2, in-situ perfusion and other artificial membrane methods seems comparably low. The P(e) and f(a) in humans for compounds that undergo mainly active transport were predicted poorly by all models investigated. However, the rat in-situ perfusion model appears useful for prediction of active uptake potential (complete active uptake is generally well predicted), and Caco-2 cells are useful for studying bidirectional active transport, respectively. Human intestinal in-vitro P(e), which correlates well with f(a) for passively transported compounds, could possibly also have potential to improve/enable predictions of f(a) for actively transported substances. Molecular descriptor data could give an indication of the passive absorption potential. The 'maximum absorbable dose' and 'dose number' approaches, and solubility/dissolution data obtained in aqueous media, appear to underestimate in-vivo dissolution to a considerable extent. Predictions of in-vivo dissolution should preferably be done from in-vitro dissolution data obtained using either real or validated simulated GI fluids.

Solubility and dissolution profile assessment in drug discovery

The purposes of the review are to: a) Provide a comprehensible introduction of the-state-of-the-art sciences of solubility and dissolution, b) introduce typical technologies to assess solubility and dissolution, and c) propose the best practice strategy. The theories of solubility and dissolution required in drug discovery were reviewed especially from the view point of oral absorption. The physiological conditions in the gastrointestinal fluid in humans and animals were then briefly summarized. Technologies to assess solubility and dissolution in drug discovery were then introduced. Recently, these technologies have been improved by the laboratory automation and computational technologies. Finally, the strategies to apply these technologies for a drug discovery project were discussed.

A formulation comparison, using a solution and different nanosuspensions of a poorly soluble compound

The pharmacokinetic parameters of AZ68 administered as a solution have been compared with those from an amorphous and a crystalline nanosuspension using rats as in vivo specie. All formulations were administered intravenously (i.v.) and orally. The purpose of the study was to find out if the three different formulations were comparable and safe to administer. The results indicate that AZ68 is absorbed at a lower rate for crystalline nanosuspensions compared to amorphous nanosuspensions and solutions. However, the absorbed extent of the compound is similar. The results are a consequence of the lower solubility and the slower dissolution rate for crystalline material compared to amorphous substance in the gastrointestinal tract. The dissolution process is excluded for a solution, resulting in the fastest absorption rate. No significant difference was found between pharmacokinetic parameters when comparison was made between the formulations after i.v. administration. There were no adverse events observed after i.v. administration of the nanosuspensions.

High throughput solubility measurement in drug discovery and development

Measurement of drug solubility in various solvents is one of the key elements of compound characterization during the whole discovery and development process. This review summarizes current experimental approaches and addresses recent advances in the experimental methods used to determine drug solubility in drug discovery and early development. This paper focuses on high throughput methods designed to determine kinetic and thermodynamic (equilibrium) solubility but traditional methods are also presented. The focus, positioning, experimental setup, pros and cons, and limitations of individual assays are discussed and differences in solubility studies in discovery and development environments are highlighted. Finally, future needs and trends in solubility assay development designed to overcome current bottlenecks and trade-offs between speed and quality/quantity of measurements are addressed.