Brain E G, Yu L J, Gustafsson K, Drewes P, Waxman D J
Department of Biology, Boston University, MA 02215, USA.
Br J Cancer. 1998 Jun;77(11):1768-76. doi: 10.1038/bjc.1998.295.
The anti-cancer prodrug ifosfamide (IF) is metabolized by liver P450 enzymes by two alternative pathways. IF is activated to 4-hydroxy IF (4-OH-IF), which ultimately yields the alkylating mustard isophosphoramide, whereas IF N-dechlororethylation inactivates the drug and produces the neurotoxic metabolite chloroacetaldehyde (CA). Both reactions are catalysed by multiple liver P450 enzymes in vitro in isolated rat liver microsomes. The present pharmacokinetic study investigates the potential for modulation of these alternative pathways of IF metabolism in vivo using the adult male Fischer 344 rat model. Rats were treated with IF alone or in conjunction with various P450 inducers and inhibitors in an effort to improve the balance between drug activation and drug inactivation. Plasma concentrations, areas under the curve (AUC) and half-lives were calculated for 4-OH-IF and CA, allowing estimations of the extent of IF activation and deactivation/toxification. Induction of liver P450 2B enzymes by 4-day high-dose phenobarbital (PB) pretreatment significantly decreased the fraction of IF undergoing 4-hydroxylation (AUC(4-OH-IF)/AUC(4-OH-IF)+AUC(CA)), from 37% to 22% of total metabolism (P < 0.05), consistent with in vitro findings that the PB-inducible P450 enzyme 2B1 plays a major role in IF N-dechloroethylation. Pretreatment with the P450 3A inducer dexamethasone proportionally decreased the AUC for both IF metabolites, without any net impact on the fraction of IF undergoing metabolic activation. By contrast, the P450 2B1 inhibitor metyrapone preferentially increased the AUC for the 4-hydroxylation pathway in 3-day low-dose PB-induced rats, thereby increasing the total fraction of IF metabolized via the activation pathway from 36% to 54% (P < 0.05), whereas the P450 inhibitors orphenadrine and troleandomycin had no significant affect on AUC values. These findings demonstrate specific roles for P450 2B and 3A enzymes in catalysing these pathways of IF metabolism in vivo, and demonstrate the potential for modulation of IF's alternative metabolic pathways in a therapeutically useful manner. These studies also highlight several clinically relevant drug interactions that may occur during concomitant administration of IF with drugs and other compounds that modulate hepatic P450 enzyme levels.
抗癌前体药物异环磷酰胺（IF）通过两条替代途径由肝脏P450酶代谢。IF被激活为4-羟基异环磷酰胺（4-OH-IF），最终产生烷基化芥子气异磷酰胺，而IF的N-去氯乙基化使药物失活并产生神经毒性代谢物氯乙醛（CA）。在体外分离的大鼠肝微粒体中，这两种反应均由多种肝脏P450酶催化。本药代动力学研究使用成年雄性Fischer 344大鼠模型，调查了体内调节IF代谢这些替代途径的可能性。大鼠单独接受IF治疗或与各种P450诱导剂和抑制剂联合使用，以努力改善药物激活与药物失活之间的平衡。计算了4-OH-IF和CA的血浆浓度、曲线下面积（AUC）和半衰期，从而可以估计IF激活和失活/毒性化的程度。4天高剂量苯巴比妥（PB）预处理诱导肝脏P450 2B酶，显著降低了经历4-羟基化的IF比例（AUC(4-OH-IF)/AUC(4-OH-IF)+AUC(CA)），从总代谢的37%降至22%（P<0.05），这与体外研究结果一致，即PB诱导的P450酶2B1在IF的N-去氯乙基化中起主要作用。用P450 3A诱导剂地塞米松预处理，按比例降低了两种IF代谢物的AUC，对经历代谢激活的IF比例没有任何净影响。相比之下，P450 2B1抑制剂美替拉酮优先增加了3天低剂量PB诱导大鼠中4-羟基化途径的AUC，从而使通过激活途径代谢的IF总比例从36%增加到54%（P<0.05），而P450抑制剂苯海拉明和醋竹桃霉素对AUC值没有显著影响。这些发现证明了P450 2B和3A酶在体内催化IF这些代谢途径中的特定作用，并证明了以治疗有用的方式调节IF替代代谢途径的可能性。这些研究还强调了在IF与调节肝脏P450酶水平的药物和其他化合物同时给药期间可能发生的几种临床相关药物相互作用。
