Pharmacokinetics and metabolism of a selective PDE5 inhibitor (UK-343,664) in rat and dog.

1. UK-343,664 is a novel potent and selective PDE5 inhibitor. Plasma clearances in the male and female rat were high (120 and 54 ml min(-1) kg(-1)), giving rise to short elimination half-lives (0.2 and 0.3h respectively). Lower clearance in dog (14 ml min(-1) kg(-1)) was the primary factor resulting in a longer elimination half-life (3.7 h). The higher clearance in rat than dog was in agreement with in vitro metabolism rates in hepatic microsomes. 2. The volume of distribution was lower in rat (1.3-2.11 kg(-1)) compared with dog (4.61 kg(-1)) probably due to increased plasma protein binding in rat (96 versus 81% in dog). 3. Oral bioavailabilities were 2, 12 and 70% in the male and female rat and dog respectively. Tmax < or = 0.5 h in all animals. 4. In multiple oral dose studies, increased systemic exposure was seen with increasing dose up to doses of 200 mg kg(-1) in rat and 150 mg kg(-1) in dog. A marked super-proportional increase in the male rat indicated a capacity-limited clearance at high doses. 5. At the maximal dose of 200 mg kg(-1) in the female rat, no clinical signs were observed after 14 days of treatment. Only minimal signs were recorded in the male rat and dog at the highest dose levels investigated. 6. After single oral or intravenous doses of [14C]-UK-343,664, the majority of radioactivity was excreted in the faeces of both species. 7. UK-343,664 was extensively metabolized in both rat and dog. The major primary pathways in dog involved piperazine N-deethylation and loss of a two carbon fragment from the piperazine ring (N,N'-de-ethylation). More extensive metabolism in the rat included additional notable metalbolites arising from hydroxylation and lactamization of the piperazine ring, which were only minor metabolites in the dog.

Synthetic strategies to lower affinity for CYP2D6.

There are several models for the CYP2D6 active site with the characteristics of their substrates and inhibitors well defined. Imipramine possesses such characteristics and is both a substrate and an inhibitor of the CYP2D6 enzyme. Possible synthetic strategies to avoid interaction with the enzyme have been investigated, including: attenuation of basicity; and alteration of rigidity and length of the alkyl chain. Imipramine inhibited the 1'-hydroxylation of bufuralol (10 microM), an in vitro marker of CYP2D6 activity, in a CYP2D6 cell line (IC50 = 2.4 microM). Inhibitory potency was attenuated by the removal of the basic centre; imipramine N-oxide had no inhibitory effect on bufuralol 1'-hydroxylation. However, removal of this basic centre, as a strategy to decrease CYP2D6 interaction, may well have a detrimental effect on pharmacological efficacy. Both an increase and decrease in the N-N carbon chain length [2C,4C] caused a reduction in inhibitory potency. In addition, introduction of a carbonyl adjacent to the amino dibenzyl moiety into 2C, 3C and 4C compounds brought about a further reduction in inhibitory potency. These data demonstrate that changes to the molecule, distal to the basic centre, can attenuate the affinity of the molecule for CYP2D6 and are in keeping with the known characteristics of the enzyme.

An investigation of the interaction between halofantrine, CYP2D6 and CYP3A4: studies with human liver microsomes and heterologous enzyme expression systems.

1. We have assessed the interaction of the antimalarial halofantrine with cytochrome P450 (CYP) enzymes in vitro, with the use of microsomes from human liver and recombinant cell lines. 2. Rac-halofantrine was a potent inhibitor (IC50 = 1.06 microM, Ki = 4.3 microM) of the 1-hydroxylation of bufuralol, a marker for CYP2D6 activity. Of a group of structurally related antimalarials tested, only quinidine (IC50 = 0.04 microM) was more potent. 3. Microsomes prepared from recombinant CYP2D6 and CYP3A4 cell lines were shown to catalyse halofantrine N-debutylation. 4. The metabolism of halofantrine to its N-desbutyl metabolite by human liver microsomes showed no correlation with CYP2D6 genotypic or phenotypic status and there was no consistent inhibition by quinidine. 5. The rate of halofantrine metabolism showed a significant correlation with both CYP3A4 protein levels (r = 0.88, P = 0.01) and the rate of felodipine metabolism (r = 0.86, P = 0.013), a marker substrate for CYP3A4 activity. Inhibition studies showed that ketoconazole is a potent inhibitor of halofantrine metabolism (IC50 = 1.57 microM). 6. In conclusion, we have demonstrated that halofantrine is a potent inhibitor of CYP2D6 in vitro and can also be metabolised by the enzyme. However, in human liver microsomes it appears to be metabolised largely by CYP3A4.