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Fig. 7.6 Structures of metoprolol and betaxolol an analogue designed to be more metabolically stable.
80 | 7 Metabolic (Hepatic) Clearance
Beside the actual steric bulk of the substituent, cyclopropyl is much more stable to hydrogen abstraction than other alkyl functions and represents an ideal terminal group. These changes make betaxolol a compound with much improved pharmacokinetics compared to its lipophilic analogues.
Catalytic Selectivity of CYP2C9
Substrates for CYP2C9 include many non-steroidal anti-inflammatory drugs plus a reasonably diverse set of compounds including phenytoin, (S)-warfarin and tolbutamide. All the substrates with routes of metabolism attributable to CYP2C9 have hydrogen bond donating groups a discrete distance from a lipophilic region which is the site of hydroxylation. The hydrogen bond donating groups and sites of metabolism on each of the substrates have been overlaid with those of phenytoin to produce a putative template of the active site of CYP2C9 (Figure 7.7).
Fig. 7.7 Template model of CYP2C9; Y is the site of oxidation, a is the distance from Y to a heteroatom which can act as a H-bond donor and c defines the angle of the H-bond.
The mean dimensions (± SD) for the eight compounds (a = 6.7 ± 0.8 A, C = 133 ± 20°) illustrates the degree of overlap achieved. Like CYP2D6 the catalytic selectivity of CYP2C9 is dominated by substrate-protein interactions.
Tolbutamide (Figure 7.8) is metabolized via the benzylic methyl group by CYP2C9 as the major clearance mechanism. Chlorpropamide is a related compound incorporating a chlorine function in this position. The resultant metabolic stability gives chlorpropamide a lower clearance and a longer half-life (approximately 35 h compared to 5 h) than tolbutamide, resulting in a substantial increase in duration of action .
Fig. 7.8 Structures of tolbutamide and the metabolically more stable analogue chlorpropamide.
The mechanism of action of CYPs is radical rather than electrophilic and the actual substitution pattern is important: the role of chlorine is one of blocking rather than deactivation. Many non-steroidal anti-inflammatory drugs are substrates for the
7.2 Cytochrome P450 81
CYP2C9 enzyme and analogous structures show how metabolic stability to p-hydrox-ylation is achieved with only small changes in substitution.
Diclofenac, with ortho substitution in the aromatic ring (Figure 7.9) is metabolized principally to 4-hydroxydiclofenac by CYP2C9. In man, the drug has a short half-life of approximately 1 h due to the relatively high metabolic (oxidative) clearance. In contrast, the analogous compound, fenclofenac, is considerably more meta-bolically stable, due to the p-halogen substitution pattern, and exhibits a half-life of over 20 h .
Catalytic Selectivity of CYP3A4
CYP3A4 attacks lipophilic drugs in positions largely determined by their chemical lability: that is, the ease of hydrogen or electron abstraction. CYP3A4 SAR is dominated therefore by substrate-reactant interaction. Binding of substrates seems to be essentially due to lipophilic forces and results in the expulsion of water from the active site. Such an expulsion of water provides the driving force for the spin state change and hence the formation of the (FeO)3+ unit. However, the lipophilic forces holding the substrate in the active site are relatively weak (~ 1 kcal mole-1) and would allow motion of the substrate in the active site. Hence, since the substrate is able to adopt more than one orientation in the active site, the eventual product of the reaction is a product of the interaction between one of these orientations and the (FeO)3+ unit - a substrate-reactant interaction. This lack of apparent substrate structure similarity (apart from chemical reactivity) indicates a large active site that allows substrate molecules considerable mobility. The selectivity of CYP3A4 to its substrates may also be directed by the conformation they adopt within a lipophilic environment such as we are suggesting for the access channel and active site of CYP3A4. We have previously illustrated this point with cyclosporin A. In an aprotic (lipophilic) solvent cyclosporin A adopts a conformation which allows the major allylic site of CYP3A4 metabolism to extend out away from the bulk of the molecule. This is a different conformation from the one adopted in aqueous solution where the lipophilic sites are internalized and thus shielded from the solvent. As a general rule this “spreading out” of apparently sterically hindered molecules as judged by X-ray or aqueous solution structure, may help to further understand the selectivity of CYP3A4. The principle of extension of lipophilic functions normally hidden from solvent is further supported by the
82 7 Metabolic (Hepatic) Clearance
Fig. 7.10 Substrates for CYP3A4 illustrating quinidine; C, L-696229; D, indinavir; E,
the diversity of structure and the “selectivity” lovastatin; F, D-THC; G, zatosetron H,