Excipient concentrations in the gastrointestinal luminal fluids can influence the absorption of poorly water-soluble drugs when dosed orally with solubilizing excipients, and poorly permeable drugs when dosed with permeation-enhancing excipients. This report examines how dose volume, excipient dose level, volume of water chaser, and gastric fluid volume influence luminal excipient concentrations, and how these could differ from preclinical species and humans. Gastric concentrations of excipient resulting immediately after dosing typical formulations containing a solubilizing excipient are estimated in preclinical species and humans. Examples of the effects of excipient dose and dose volume on drug absorption are illustrated using cases in the literature. When estimating human absorption potential of poorly soluble drug candidates from solubility data, in vitro dissolution models, physiologically based pharmacokinetic models, or preclinical pharmacokinetic data, it may be useful to consider the dose volume and excipient dose, and it is recommended to include estimated luminal excipient concentrations as a factor. There is a need for further studies collecting data on excipient concentrations in luminal fluids and evaluating the effects on drug absorption.

For discovery teams working toward new, orally administered therapeutic agents, one requirement is to attain adequate systemic exposure after oral dosing, which is best accomplished when oral bioavailability is optimized. This report summarizes the bioavailability challenges currently faced in drug discovery, and the design and testing methods and strategies currently utilized to address the challenges. Profiling of discovery compounds usually includes separate assessments of solubility, permeability, and susceptibility to first-pass metabolism, which are the 3 most likely contributors to incomplete oral bioavailability. An initial assessment of absorption potential may be made computationally, and high throughput in vitro assays are typically performed to prioritize compounds for in vivo studies. The initial pharmacokinetic study is a critical decision point in compound evaluation, and the importance of the effect the dosing vehicle or formulation can have on oral bioavailability, especially for poorly water soluble compounds, is emphasized. Dosing vehicles and bioavailability-enabling formulations that can be used for discovery and preclinical studies are described. Optimizing oral bioavailability within a chemical series or for a lead compound requires identification of the barrier limiting bioavailability, and methods used for this purpose are outlined. Finally, a few key guidelines are offered for consideration when facing the challenges of optimizing oral bioavailability in drug discovery.

Transdermal dosage forms provide controlled drug dosing through the skin from bandage-like delivery systems. This method of drug delivery can provide certain medical advantages for many drugs. The skin is a very good barrier preventing the absorption of such foreign substances; therefore, most drugs don't diffuse through it rapidly enough for ordinary formulations to be administered transdermally. Skin permeation enhancers increase skin permeability and may make it feasible to deliver many more drugs transdermally. In addition, they may be useful for improving the efficacy of poorly permeating, topically acting drugs. The literature on skin permeation enhancers is reviewed, with particular emphasis on structure/activity relationships, mechanisms of enhancing skin permeability, and toxicity of skin permeation enhancing agents. Although this summary may not enable one to predict how to effectively and safely enhance transdermal or dermal delivery for some given drug, it should provide an understanding of the studies that are important to reaching such a target. As an introduction, the properties of skin as a barrier and the pathways of transport across the skin are discussed.

Absorption enhancers are functional excipients included in formulations to improve the absorption of a pharmacologically active drug. The term absorption enhancer usually refers to an agent whose function is to increase absorption by enhancing membrane permeation, rather than increasing solubility, so such agents are sometimes more specifically termed permeation enhancers. Absorption enhancers have been investigated for at least two decades, particularly in efforts to develop non-injection formulations for peptides, proteins, and other pharmacologically active compounds that have poor membrane permeability. While at least one product utilizing an absorption enhancer for transdermal use has reached the market, quite a few more appear to be at the threshold of becoming products, and these include oral and transmucosal applications. This paper will review some of the most advanced absorption enhancers currently in development and the formulation technologies employed that have led to their success. In addition, a more basic review of the barriers to absorption and the mechanisms by which those barriers can be surmounted is presented. Factors influencing the success of absorption-enhancing formulations are discussed. If ultimately successful, the products now in development should offer non-injection alternatives for several peptide or protein drugs currently only administered by injection. The introduction of new absorption enhancers as accepted pharmaceutical excipients, and the development of formulation technologies that afford the greatest benefit/risk ratio for their use, may create opportunities to apply these enabling technologies more broadly to existing drugs with non-optimal delivery properties.

The pharmacokinetic (PK) repeat study sample, selected by the study pharmacokineticist, requires repeat bioanalysis because the concentration is incongruous with drug plasma concentration versus time profile. The inconsistency could be due to a number of reasons, including the detectable drug concentration in a predose sample or a sample from a placebo control group or a significant double peak in the terminal phase of an individual plasma concentration versus time profile that is not consistent with the profiles from other subjects in the same dose group. The justification for selecting the PK repeat sample should be clearly documented. The repeat analysis should be conducted in duplicate or triplicate as allowed by sample volume. To avoid subjectively selecting PK repeat samples, standard operating procedures should be prepared prior to the start of the study in order to define the criteria for selecting PK repeat study samples and also the procedure for conducting repeat analysis and reporting repeat assay values. The incurred sample re-analysis (ISR) assessment and the repeat analysis of pharmacokinetically anomalous samples are different in terms of purpose and conduct; the ISR assessment alone cannot accept or reject the results from a study for analytical reasons. Therefore, the results from the ISR assessment for assuring the reliability and reproducibility of a validated bioanalytical method in animal or human plasma or other biological matrices should not be used to substitute the results of repeat analysis of pharmacokinetically anomalous samples from a nonclinical or clinical study.

Inhibition of dopamine reuptake via the dopamine transporter (DAT) present on CNS presynaptic dopaminergic neurons is a potential therapy for the treatment of Parkinson's disease (PD). DAT blockers have previously been shown to mitigate PD-like symptoms induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in nonhuman primates. This study evaluated brain penetration of three DAT-selective blockers, O-620 (difluoropine), O-1369, and O-2099, in mice after administration of single oral doses. DAT blockers were administered in aqueous phosphate buffer (pH 3.0) by oral gavage at doses of 2, 5, 10, and 20 mg/kg to four groups of mice. Blood samples and whole brain specimens were taken at 0.5, 1, 2, 4, 6, 8, and 24 hours after the dose. Blood plasma and brain DAT blocker content used validated LC/MS/MS assay methodology. Pharmacokinetics analyses to determine elimination half-lives and area under the concentration vs. time (AUC) curves for plasma and brain were performed using WinNonlin software (Pharsight, Mountain View, CA). All three DAT blockers achieved rapid systemic and brain exposures (Cmax and AUC) with maximal plasma and brain concentrations obtained within 0.5 hours. Half-lives ranged from 0.3 to 2.7 hours (plasma) and 0.3 to 3.3 hours (brain). Plasma mean Cmax values after doses of 2, 5, 10, and 20 mg/kg were 49.4, 128, 579, and 1338 ng/mL (O-620); 12, 19, 73, and 417 ng/mL (O-1369); and 172, 406, 759, and 1509 ng/mL (O-2099). Corresponding brain mean Cmax values were 377, 1547, 3348, and 9407 ng/g (O-620); 60, 131, 497, and 2584 ng/g (O-1369); and 447, 1049, 2594, and 6883 ng/g (O-2099). Mean brain-to-plasma concentration ratios and mean brain-to-plasma AUC ratios ranged from 2.2 to 17.6 and from 6.4 to 10.3, respectively, for O-620; from 5.3 to 8.3 and from 6.7 to 8.3, respectively, for O-1369; and from 2.2 to 17.7 and from 6.4 to 10.3, respectively, for O-2099. DAT blockers have previously been shown to mitigate PD-like symptoms induced by MPTP in nonhuman primates. Here, we show that oral administration in mice of three DAT-selective blockers resulted in rapid absorption into the systemic circulation and the CNS. The CNS bioavailability and favorable brain-to-plasma ratios after oral dosing makes these DAT blockers attractive candidates for treatment of PD. The work described here was conducted by QPS, L.L.C., for Alseres Pharmaceuticals. The compounds tested were synthesized for Alseres Pharmaceuticals by Organix, Inc.

Background and Objective An assessment of clinically important change is useful in interpreting clinical trial outcomes. The 4-point Ashworth Scale (AS) is a widely accepted measure of muscle tone, and the 9-point Physician Global Assessment Score (PGAS) is a global measure of post-treatment change as evaluated by the physician. The quantitative relationship between AS and PGAS has not previously been described in spasticity. Methods Data from three clinical trials of botulinum toxin type A (BOTOX; Allergan, Irvine, CA) in poststroke spasticity were analyzed ( n = 442). Mean change from baseline in AS score was plotted as a function of PGAS and correlations were calculated. Receiver–operator curve (ROC) analyses with the wrist flexor AS change from baseline as the independent variable and PGAS as the dependent variable were performed using the following criteria: PGAS of ≥1 (mild improvement or better) and PGAS of ≥2 (moderate improvement or better). Results In pooled data, the Pearson's correlation coefficient r between the change in wrist Ashworth Score and the PGAS was −0.44 ( p = 0, t = −26.5). Pearson correlations by individual study were also statistically significant. By ROC analysis, a PGAS of ≥1 was associated with 33% reduction of wrist AS and a PGAS of ≥2 was associated with approximately 50% reduction. In a well-powered phase 3 study, the Pearson's correlation coefficient was even higher ( r = −0.76, p = 0, t = −28.5). Conclusions Changes in disease-specific scales that correlate significantly with changes in physician global assessments are considered clinically meaningful. In clinical trials of chronic spasticity, we found that 33% and 50% changes in the Ashworth Scale scores correlate with 1-point or 2-point changes in PGAS. This is similar to findings from pooled chronic pain studies, in which 30% and 50% changes on the pain numeric rating scale were considered clinically important (Farrar et al., Pain 2001;94:149–158). Study supported by Allergan, Irvine, CA.

AIMS Itopride, a new prokinetic drug, is highly bound to plasma proteins. This study aimed at assessing the influence of itopride on the protein binding of two drugs with narrow therapeutic indices: digoxin and warfarin. METHODS In vitro protein binding in human plasma was determined using an equilibrium dialysis method after incubation at 37°C for 3 hours for warfarin and 6 hours for digoxin. Drugs were assayed in plasma and phosphate buffer by LC/MS/MS. Itopride and digoxin were tested at concentrations close to low, middle, and high therapeutic range, namely: 100 ng/mL, 250 ng/mL and 500 ng/mL for itopride and 0.8 ng/mL, 1.7 ng/mL and 2.5 ng/mL for digoxin. Warfarin was tested at concentration close to low and high therapeutic range, 1.8 μg/mL and 2.6 μg/mL. Each value is the mean of three assays. RESULTS Mean (S.D.) % of free digoxin or warfarin in human plasma at equilibrium (See Table) Itopride (ng/ mL) Digoxin (0.8 ng/ mL) Digoxin (1.7 ng/ mL) Digoxin (2.5 ng/ mL) Warfarin (1.8 μg/ mL) Warfarin (2.6 μg/ mL) 0 99.3 (23.4) 71.4 (9.1) 81.5 (32.8) 1.4 (0.4) 1.7 (0.8) 100 120.0 (22.4) 92.8 (1.3) 79.2 (3.5) 2.6 (1.0) 2.4 (1.2) 250 107.5 (41.7) 80.0 (8.2) 90.7 (13.6) 2.0 (1.0) 1.7 (0.8) 500 78.3 (12.2) 90.0 (14.0) 73.4 (14.8) 1.9 (0.7) 1.7 (0.8) CONCLUSIONS These data suggest that itopride is not at risk of producing a clinically significant drug-drug interaction with digoxin. The clinical relevance of the increase seen with warfarin at itopride concentration of 100 ng/mL remains to be clarified. Clinical Pharmacology & Therapeutics (2005) 79, P26–P26; doi: 10.1016/j.clpt.2005.12.095

The significance of intestinal P-glycoprotein (P-gp) in determining the oral bioavailability of tacrolimus has been still controversial. In this study, we reevaluated the interaction of tacrolimus with P-gp in the rat small intestine, by evaluating its absorption from the rat small intestine and its modulating effect on the absorption of known P-gp substrates (digoxin, methylprednisolone, and vinblastine). Intestinal absorption of tacrolimus itself was as extensive as other P-gp modulators such as cyclosporine and verapamil. While cyclosporine and verapamil significantly increased the absorption of methylprednisolone and vinblastine through potent inhibition of intestinal P-gp, tacrolimus failed to achieve this. When cyclosporine and tacrolimus were intravenously administered to rats, digoxin absorption was significantly increased by cyclosporine but not by tacrolimus. When tacrolimus was coadministered with clotrimazole, a specific CYP3A inhibitor, into the rat small intestine, the area under the curve of tacrolimus blood concentrations increased more than seven-fold compared with that of tacrolimus alone. Our present results strongly suggest that the interaction between tacrolimus and P-gp is limited in the rat small intestine and that extensive metabolism by CYP3A enzymes is more responsible for the low oral bioavailability of tacrolimus. It was considered that the extensive absorption of cyclosporine and verapamil was closely associated with their potent ability to inhibit intestinal P-gp.

PURPOSE:Loperamide-induced suppressive effects on central nervous system closely relate to a lack of or decline in the P-glycoprotein (P-gp) function. The aim of this study was to determine the loperamide-induced sedative effect quantitatively and to investigate possible alterations in the pharmacokinetics of digoxin, a substrate for P-gp, in Japanese subjects.METHODS:Loperamide hydrochloride (2 mg) was administered orally to 26 subjects and the critical flicker-fusion frequency threshold (CFF) values were measured every 30 min separately by portable instrument. Further, digoxin (0.25 mg) was administered to 8 subjects, and the plasma concentration was determined.RESULTS:In five subjects who complained of drowsiness, the CFF values more remarkably decreased compared with those in the other subjects. The Tmax and mean residence time (MRT) values of digoxin pharmacokinetics in four subjects with drowsiness were significantly lower and Cmax was higher than those in four subjects with marginal effect. Moreover, there were good correlations between the CFF value-time profile and the Cmax, Tmax, and MRT of digoxin.CONCLUSIONS:The determination of the CFF value after oral administration of loperamide will be useful for evaluating varied P-gp function and for anticipating individual variations in the disposition of P-gp substrates in humans.

Purpose.The purpose of this study was to develop and validate a method for separately evaluating the roles of gastrointestinal absorption and hepatic extraction as barriers to oral bioavailability (BA). The method was validated using five reference compounds known to have different absorption and hepatic extraction properties. Dose-dependence was also investigated for one reference compound.

To evaluate the intestinal permeability of poorly water-soluble compounds, it is of importance to completely dissolve them in a medium and to avoid precipitation during experiments. This study was undertaken to find an agent possessing a high-solubilizing capacity and exhibiting minimal modulating impact on membrane integrity and absorption systems such as passive diffusion and carrier-mediated permeation. Phenytoin dissolution was compared in the presence of seven solubilizing agents at concentrations of 1, 2, or 5% using a centrifugation method. The capacity to dissolve phenytoin was great in β-cyclodextrin (β-CD) and hydroxypropyl β-cyclodextrin, followed by Tween 80. Those of methanol, dimethyl sulfoxide, dimethyl acetoamide, and polyethylene glycol 400 were much lower than expected. One percent β-CD did not alter the absorption of fluorescein isothiocyanate-dextran 4000 or the release of protein and lactate dehydrogenase into in situ loop contents, suggesting that 1% β-CD had no significant impact on the integrity of the intestinal membrane. One percent β-CD also did not alter the absorption of caffeine, ceftibuten, or rhodamine 123 from in situ jejunal loops, indicating no interference with passive diffusion and active transports mediated by a peptide transporter and P-glycoprotein. In conclusion, 1% β-CD is a suitable solubilizing agent for evaluating in situ intestinal absorption of poorly water-soluble compounds.