Drug Metabolism Letters - Volume 5, Issue 4, 2011

Fluorine- and chlorine-containing moieties have been strategically integrated into chemical structures to optimize the pharmacokinetic and metabolic properties of therapeutic agents, based partly on the concept that the addition of these substituents may lower microsomal clearance. A large-scale systematic mechanistic study of drug metabolic alteration by aromatic halogenation has hitherto not been possible due to the lack of either large clearance databases or adequate data mining tools. To address this, we systematically searched compound pairs in Pfizer's human liver microsomal clearance database of over 220,000 unique compounds to assess the effects of fluoro-, chloro- and trifluoromethyl-substitution on phenyl derivatives. Although the para-position fluorination and chlorination lowered the microsomal clearance statistically, the substitution at the ortho and meta positions for the studied fluorine- and chlorinecontaining moieties dramatically increased the microsomal clearance. More importantly, we found that changes in physicochemical properties, electronic properties, and specific binding of substrates to drug metabolizing enzymes, for instance, cytochrome P450s, are all determining factors that drive the direction of microsomal clearance when a specific series of compounds are studied.

The effect of oxidative stress (OS) on the pharmacokinetics of clomipramine (CPM), particularly addressing the change of CPM distribution to plasma components, was studied in ferric-nitrilotriacetate-induced oxidative-stress model rats (OS rats). First, CPM pharmacokinetic studies in OS rats were performed using CPM continuous infusion (17.5 μg/min/kg). Plasma concentration of CPM at a steady state in OS rats (0.20 ± 0.02 μg/mL) was significantly lower than that in control rats (0.30 ± 0.02 μg/mL). However, no difference was found in the amounts of CPM in the brains of control rats (1.67 ± 0.13 μg/g) and OS rats (1.63 ± 0.09 μg/g). Both of plasma unbound fraction and distribution to erythrocytes in OS rats were significantly higher than those of control rats. These results suggest that the lower CPM concentration in plasma in OS condition does not induce an inferior pharmacological effect.

In contrast to human CYP2C9, non-human CYP2C enzymes do not appear to preferentially bind and metabolize anionic drugs. Using analogs of sulfaphenazole, the effect of an acidic sulfonamide group on apparent affinity and turnover rates was characterized with canine CYP2C21. Blocking the sulfonamide with a methyl group increased the intrinsic clearance by CYP2C21 >100-fold and decreased Km. Furthermore, CYP2C21 demonstrated selectivity for formation of the benzylic hydroxylation product and a high estimated fm,CYP value. The findings suggest that canine CYP2C21, unlike human CYP2C9, does not derive ligand binding affinity from an anion binding interaction with sulfaphenazole analogs.

The luminol/sulfobutylether-β-cyclodextrin (SBE7M-β-CD) chemiluminescence (CL) system and the interaction of SBE7M-β-CD/rutin were first described by flow injection (FI) CL method. It was found that SBE7M-β-CD with luminol could form 1:1 complex online, which could accelerate the electrons transferring rate of excited 3-aminophthalate, giving the enhanced CL intensity of luminol. The enhancement of CL intensity was proportional to the concentrations of SBE7M- β-CD with a linear range from 25 to 1750 μmol L-1. It was also found that rutin could inhibit the CL intensity from luminol/SBE7M-β-CD system, and the decrement of CL intensity was logarithm over the concentrations of rutin ranging from 0.1 to 100.0 nmol L-1, giving the regression equation ΔI = 32.90lgCrutin + 16.26 (R2 = 0.9952) with a detection limit of 0.03 nmol L-1 (3σ). According to the proposed CL model, the binding constant (KCD-R) and the stoichiometric ratio of SBE7M-β-CD/rutin complex were obtained as 1.6 × 106 L2 mol-2 and 2:1. The possible mechanism of luminol/SBE7M-β- CD/rutin interaction was also discussed. The method was successfully applied to monitor rutin in human urine samples after ingesting SBE7M-β-CD/rutin complex, with a total excretion of 68.8% within 8.0 h.

Poor pharmacokinetics and toxicity are responsible for most drug candidate failures. In order to attempt to some degree of ADMET (Absorption, Distribution, Metabolism, Excrection and Toxicity) information, in silico predictions arise currently as an interesting alternative to evaluate prototypes during early stages of the drug design processes, especially for anticancer candidates that constitute a class of therapeutic agents that exhibit substantial toxicity. A benzimidazole and a phenylbenzamide derivatives, previously identified as novel anticancer lead compounds able to prevent DNA binding to hnRNP K protein, were evaluated in silico regarding their metabolic profile and toxicity potential in order to give insights to the design of drug candidates with an adequate pharmaceutical profile. Considering the structure of proposed metabolites for both molecules, the phenylbenzamide derivative seems to be a molecule with better pharmaceutic profile, since its possible metabolites present a milder degree of chemical structure toxic alerts than the benzimidazole derivative that can cause chromosome damage induced by the benzimidazole group. It would be desirable during optimization of the phenylbenzamide derivative to maintain these characteristics during generation of analogues with substituents that are not known as potent toxicophoric groups. For the benzimidazole derivative, if the toxic events are really severe as it seems, one possible strategy would be replace the benzimidazole ring system by bioisosteres with lower toxic potential, hoping to maintain or enhance biological activity.

In the present study, an LC study of degradation behavior of baicalin under various buffer solutions (pH 5.0, 7.0, 9.0) was carried out during simulative blood sample processing with Solid Phase Extraction technology. The results show that during simulative blood sample processing with Solid Phase Extraction technology, baicalin was apt to undergo degradation in buffer solutions (pH 7.0, 9.0) but was stable in buffer solutions (pH 5.0). Primarily, maximum degradation product was achieved by subjecting baicalin to drug-free blood and buffer solution (pH 7.0), standing for 30 min, then rinsing with buffer solution (pH 7.0) followed by elution with methanol from Solid Phase Extraction cartridge, and keeping the methanol eluate at 50 °C for more than 11 hs. The product was characterized as negletein through LC-MS fragmentation and 1H-NMR, 13C-NMR spectra studies.

Rufinamide was evaluated in vitro to determine which enzyme(s) are responsible for rufinamide hydrolysis and whether valproate, one of its metabolites (valproyl-CoA), and/or the rufinamide hydrolysis product (CGP 47292) could inhibit hydrolysis. Rufinamide hydrolysis was mediated primarily by human carboxylesterase (hCE) 1 and was nonsaturable up to 500 μM. Two-thirds of rufinamide hydrolysis was estimated to occur in human microsomes and one-third in cytosol. Valproate was a selective inhibitor for hCE1 compared to hCE2 and inhibition had a greater impact on rufinamide hydrolysis in microsomes than in cytosol. Valproyl-CoA caused similar inhibition of rufinamide hydrolysis in both microsomes and cytosol. Carboxylesterases were not significantly inhibited by CGP 47292. Inhibition of in vitro rufinamide hydrolysis by valproate could offer an explanation for the observed in vivo drug-drug interaction between the two antiepileptic drugs.

AMG 900 is an orally available small molecule that is highly potent and selective as a pan-aurora kinase inhibitor. AMG 900 is currently undergoing phase 1 clinical evaluation in patients with advanced solid tumors. The metabolism of AMG 900 was investigated in both male and female rats. We conducted studies in bile-duct catheterized (BDC) rats where bile, urine and plasma were analyzed to obtain metabolism profiles for each gender. These studies identified gender differences in the metabolism profiles in bile. Bile contained the majority of the drug related material and contained little unchanged AMG 900 which indicated that metabolism was the prominent process in drug elimination. Although bile contained the same metabolites for both genders, the amount of specific metabolites differed. Male rats metabolized AMG 900 primarily through hydroxylation with subsequent sulfate conjugation on the pyrimidinyl-pyridine side-chain whereas female rats favored a different oxidation site on the thiophene ring's methyl group, which is then metabolized to a carboxylic acid with subsequent conjugation to an acyl glucuronide. CYP phenotyping identified the prominent isoforms as being gender specific or biased in the oxidative metabolism of AMG 900. The metabolism in male rats favored both CYP2C11 and CYP2A2 whereas females favored the CYP2C12. The prominent sulfate conjugate identified in the male rat bile could also be due to male biased metabolism since it has been reported that sulfate conjugation is more prevalent in male rats. All the prominent rat metabolism routes for AMG 900 either have male or female bias. These differences in the rat AMG 900 metabolism profiles in bile can be explained by gender specific P450CYP isoforms.