These experimental observations demonstrated how the thermal events were attributed to phase transitions occurring in a binary mixture and clarified the relationship between exothermic peaks and cocrystal formation.

ABSTRACT:To verify the availability of pharmacokinetic parameters in cynomolgus monkeys, hepatic availability (Fh) and the fraction absorbed multiplied by intestinal availability (FaFg) were evaluated to determine their contributions to absolute bioavailability (F) after intravenous and oral administrations. These results were compared with those for humans using 13 commercial drugs for which human pharmacokinetic parameters have been reported. In addition, in vitro studies of these drugs, including membrane permeability, intrinsic clearance, and p-glycoprotein affinity, were performed to classify the drugs on the basis of their pharmacokinetic properties. In the present study, monkeys had a markedly lower F than humans for 8 of 13 drugs. Although there were no obvious differences in Fh between humans and monkeys, a remarkable species difference in FaFg was observed. Subsequently, we compared the FaFg values for monkeys with the in vitro pharmacokinetic properties of each drug. No obvious FaFg differences were observed between humans and monkeys for drugs that undergo almost no in vivo metabolism. In contrast, low FaFg were observed in monkeys for drugs that undergo relatively high metabolism in monkeys. These results suggest that first-pass intestinal metabolism is greater in cynomolgus monkeys than in humans, and that bioavailability in cynomolgus monkeys after oral administration is unsuitable for predicting pharmacokinetics in humans. In addition, a rough correlation was also observed between in vitro metabolic stability and Fg in humans, possibly indicating the potential for Fg prediction in humans using only in vitro parameters after slight modification of the evaluation system for in vitro intestinal metabolism.Because the development of new drugs is a cost-and labor-intensive task, the selection of candidates with good pharmacokinetic profiles is becoming increasingly common. This practice minimizes the number of drug candidates dropped due to pharmacokinetic problems during the clinical phase (Wishart, 2007).When predicting human pharmacokinetics, the fraction absorbed (Fa), intestinal availability (Fg), and hepatic availability (Fh) are the main factors to consider. Fh prediction has become considerably accurate since several mathematical prediction models have been established, including the physiological model, well stirred model, parallel tube model, and dispersion model (Iwatsubo et al., 1996;Naritomi et al., 2001;De Buck et al., 2007). For FaFg, however, no quantitative prediction method has ever been established, although several qualitative prediction methods using human intestinal microsomes have been reported (Chiba et al., 1997;Shen et al., 1997;Fagerholm, 2007;Fisher and Labissiere, 2007;Yang et al., 2007). For these reasons, we have mainly used animal pharmacokinetic parameters to predict human FaFg in the drug discovery stage.It has been regarded as natural that monkey metabolism is the most similar to that of humans, and cynomolgus monkeys have been widely used in pharmacokinetic or drug-...

We describe the preclinical and clinical pharmacokinetic profiles of FK3453 [6-(2-amino-4-phenylpyrimidin-5-yl)-2-isopropylpyridazin-3(2H)-one] and the mechanism responsible for poor oral exposure of FK3453 in humans. FK3453 showed favourable profiles in preclinical pharmacokinetic studies, including satisfactory absolute bioavailability and total body clearance in animals (30.5%-41.4%, 54.7%-68.2%, and 71.3%-93.4% and 10.8-17.6, 1.9-17.1, and 5.0 mL/min/kg in male rats, female rats, and dogs, respectively), and good metabolic stability in liver microsomes (42.3, 14.5, and 1.1 mL/min/kg in male rats, dogs, and humans, respectively). However, despite these promising preclinical findings, plasma concentrations of FK3453 in humans were extremely low, with the oxidative metabolite of the aminopyrimidine moiety (M4) identified as a major metabolite. Given that aldehyde oxidase (AO) and xanthine oxidase (XO) were presumed to be the enzymes responsible for M4 formation, we investigated the mechanism of M4 formation using human liver subcellular fractions. M4 was detected in the incubation mixture with S9 and cytosol but not with microsomes, and M4 formation was inhibited by AO inhibitors (menadione, isovanillin) but not by cytochrome P-450 inhibitor (1-aminobenzotiazole) or XO inhibitor (allopurinol). These results suggest M4 formation is catalyzed by AO, and therefore, its poor exposure in humans was attributed to extensive AO metabolism.

ABSTRACT:This study aimed to establish a practical and convenient method of predicting intestinal availability (F g ) in humans for highly permeable compounds at the drug discovery stage, with a focus on CYP3A4-mediated metabolism. We constructed a "simplified F g model," described using only metabolic parameters, assuming that passive diffusion is dominant when permeability is high and that the effect of transporters in epithelial cells is negligible. Five substrates for CYP3A4 (alprazolam, amlodipine, clonazepam, midazolam, and nifedipine) and four for both CYP3A4 and P-glycoprotein (P-gp) (nicardipine, quinidine, tacrolimus, and verapamil) were used as model compounds. Observed fraction of drug absorbed (F a F g ) values for these compounds were calculated from in vivo pharmacokinetic (PK) parameters, whereas in vitro intestinal intrinsic clearance (CL int,intestine ) was determined using human intestinal microsomes. The CL int,intestine for the model compounds corrected with that of midazolam was defined as CL m,index and incorporated into a simplified F g model with empirical scaling factor. Regardless of whether the compound was a P-gp substrate, the F a F g could be reasonably fitted by the simplified F g model, and the value of the empirical scaling factor was well estimated. These results suggest that the effects of P-gp on F a and F g are substantially minor, at least in the case of highly permeable compounds. Furthermore, liver intrinsic clearance (CL int,liver ) can be used as a surrogate index of intestinal metabolism based on the relationship between CL int,liver and CL m,index . F g can be easily predicted using a simplified F g model with the empirical scaling factor, enabling more confident selection of drug candidates with desirable PK profiles in humans.Given the substantial time and cost associated with drug discovery and development, increasing importance has been placed on the prediction of pharmacokinetics (PK) in humans of drug candidates at the discovery stage to avoid later termination of development because of an undesirable PK profile. Bioavailability (F) of an orally administered drug, which is the fraction of drug reaching systemic blood circulation, is expressed as the product of the fraction of the dose that enters the enterocyte (F a ), intestinal availability (F g ), and hepatic availability (F h ). Therefore, to effectively pass into systemic blood circulation, orally administered drugs must not only have high solubility and permeability in the gastrointestinal tract but also be stable against metabolizing enzymes in the gastrointestinal tract and liver.At the drug discovery stage, compounds selected as drug candidates using high-throughput screening typically are those with high permeability because intestinal permeability is one of the most important factors in determining the F of orally administered drugs. Although P-glycoprotein (P-gp) is highly expressed in intestinal epithelial cells and has the potential to reduce drug absorption, the effect on drug absorption is not qu...

The bioavailability (F) of midazolam in cynomolgus monkeys (0.02) was markedly lower than that in humans (0.24-0.46) and the reason for this difference in F between the two species was investigated. Based on the area under the plasma concentration-time curve after intravenous and intraportal infusion to cynomolgus monkeys, the hepatic availability (F(h)) was estimated as 0.66. The fraction of dose absorbed (F(a)) estimated from the single-pass intestinal perfusion method was 1.0 in cynomolgus monkeys. The intestinal availability (F(g) = F/F(a)/F(h)) was calculated as 0.03 in cynomolgus monkeys. Since the F(a) of midazolam has been reported to be almost 1.0 in humans, F(h) and F(g) were calculated as 0.33-0.76 and 0.46-1.00 when the reference values for hepatic blood flow (1026-1530 ml h(-1) kg(-1)) were used. In conclusion, the main reason for low F in cynomolgus monkeys was the markedly higher first-pass intestinal metabolism seen in cynomolgus monkeys compared with humans.