Methylnaltrexone (MNTX) is a peripherally acting μ-opioid receptor antagonist and is currently indicated for the treatment of opioid-induced constipation in patients with advanced illness who are receiving palliative care, when response to laxative therapy has not been sufficient. Sulfation to MNTX-3-sulfate (M2) and carbonyl reduction to methyl-6α-naltrexol (M4) and methyl-6β-naltrexol (M5) are the primary metabolic pathways for MNTX in humans. The objectives of this study were to investigate MNTX in vitro metabolism in human and nonclinical species and to identify the human enzymes involved in MNTX metabolism. Of the five commercially available sulfotransferases investigated, only SULT2A1 and SULT1E1 catalyzed M2 formation. Formation of M4 and M5 was catalyzed by NADPH-dependent hepatic cytosolic enzymes, which were identified using selective chemical inhibitors (10 and 100 μM) for aldo-keto reductase (AKR) isoforms, short-chain dehydrogenase/reductase including carbonyl reductase, alcohol dehydrogenase, and quinone oxidoreductase. The results were then compared with the effects of the same inhibitors on 6β-naltrexol formation from naltrexone, a structural analog of MNTX, which is catalyzed mainly by AKR1C4. The AKR1C inhibitor phenolphthalein inhibited MNTX and naltrexone reduction up to 98%. 5β-Cholanic acid 3α,7α-diol, the AKR1C2 inhibitor, and medroxyprogesterone acetate, an inhibitor of AKR1C1, AKR1C2, and AKR1C4, inhibited MNTX reduction up to 67%. Other inhibitors were less potent. In conclusion, the carbonyl reduction of MNTX to M4 and M5 in hepatic cytosol was consistent with previous in vivo observations. AKR1C4 appeared to play a major role in the carbonyl reduction of MNTX, although multiple enzymes in the AKR1C subfamily may be involved. Human SULT2A1 and SULT1E1 were involved in MNTX sulfation.
甲基纳曲酮 (MNTX) 是一种外周作用的 μ-阿片受体拮抗剂,目前适用于治疗正在接受姑息治疗的晚期疾病患者的阿片类药物引起的便秘,而这些患者对泻药治疗的反应不充分。
硫酸化为 MNTX-3-
硫酸盐 (M2) 和羰基还原为甲基-6α-纳曲醇 (
M4) 和甲基-6β-纳曲醇 (M5) 是人类中 MNTX 的主要代谢途径。本研究的目的是研究人类和非临床物种的 MNTX 体外代谢,并鉴定参与 MNTX 代谢的人类酶。在研究的五种市售磺基转移酶中,只有 SULT2A1 和 SULT1E1 催化 M2 形成。
M4 和 M5 的形成由
NADPH 依赖性肝胞质酶催化,这些酶使用醛酮还原酶 (AKR) 亚型、短链脱氢酶/还原酶(包括羰基还原酶、
乙醇脱氢酶)的选择性
化学抑制剂(10 和 100 μM)进行鉴定和醌氧化还原酶。然后将结果与相同
抑制剂对
纳曲酮(MNTX 的结构类似物,主要由 AKR1C4 催化)形成 6β-纳曲醇的影响进行比较。 AKR1C
抑制剂酚酞可抑制 MNTX 和
纳曲酮减少高达 98%。
5β-胆烷酸 3α,7α
-二醇(AKR1C2
抑制剂)和
醋酸甲羟孕酮(AKR1C1、AKR1C2 和 AKR1C4
抑制剂)可抑制 MNTX 减少高达 67%。其他
抑制剂的效力较差。总之,肝细胞质中 MNTX 羰基还原为
M4 和 M5 与之前的体内观察结果一致。尽管 AKR1C 亚家族中的多种酶可能参与其中,但 AKR1C4 似乎在 MNTX 的羰基还原中发挥了主要作用。人 SULT2A1 和 SULT1E1 参与 MNTX
硫酸化。