Interaction of irinotecan (CPT-11) and its active metabolite 7-ethyl-10-hydroxycamptothecin (SN-38) with human cytochrome P450 enzymes.

The inhibition and mechanism-based inactivation potencies of irinotecan (7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin; CPT-11) and its active metabolite (7-ethyl-10-hydroxycamptothecin; SN-38) for human cytochrome P450 (P450) enzymes were investigated to evaluate the potential for drug interactions involving CPT-11 using microsomes from insect cells expressing specific human P450 isoforms. The mechanism and potential for interaction were examined by Lineweaver-Burk analysis, and NADPH-, time- and concentration-dependent effects were observed. CPT-11 and SN-38 competitively inhibited CYP3A4 (testosterone 6 beta-hydroxylation) activity with K(i) values of 129 and 121 microM, respectively. CYP2A6 (coumarin 7-hydroxylation) and CYP2C9 (diclofenac 4'-hydroxylation) activities exhibited a mixed type of inhibition comprising competitive and noncompetitive components in response to SN-38, the K(i) values being 181 and 156 microM, respectively. On the other hand, CYP1A2 (phenacetin O-deethylation), CYP2B6 (7-ethoxycoumarin O-deethylation), CYP2C8 (paclitaxel 6 alpha-hydroxylation), CYP2C19 (S-mephenytoin 4'-hydroxylation), CYP2D6 (bufuralol 1'-hydroxylation), and CYP2E1 (chlorzoxazone 6-hydroxylation) were hardly affected by either compound. Furthermore, CPT-11 and SN-38 were suggested to be mechanism-based inactivators of CYP3A4. The k(inact) and K(I) values of CPT-11 and SN-38 were 0.06 min(-1) and 24 microM and 0.10 min(-1) and 26 microM, respectively. However, no inactivation of CYP2A6 and CYP2C9 by SN-38 was observed. These results mean that CPT-11 and SN-38 interact with human P450 isoforms, such as CYP2A6, CYP2C9, and CYP3A4, in vitro and imply that the significant drug interactions involving CPT-11 may be caused by a mechanism-based inactivation of CYP3A4 by SN-38 as an active metabolite of CPT-11 rather than competitive inhibition.     

Glucuronidation of 7-ethyl-10-hydroxycamptothecin (SN-38), an active metabolite of irinotecan (CPT-11), by human UGT1A1 variants, G71R, P229Q, and Y486D.

7-Ethyl-10-hydroxycamptothecin (SN-38), an active metabolite of antitumor agent irinotecan (CPT-11), is conjugated and detoxified to SN-38-glucuronide by UDP-glucuronosyltransferase (UGT) 1A1. Genetic polymorphisms in UGT1A1 are thought to contribute to severe diarrhea and/or leukopenia caused by CPT-11. In this regard, it has been reported that polymorphisms in the promoter region could affect the CPT-11 pharmacokinetics and interindividual variation of toxicity. However, little information is available on the influence of UGT1A1 polymorphisms in the coding region on the SN-38 glucuronidation activity. In the present study, wild-type (WT) and three variant (G71R, P229Q, and Y486D) cDNAs of human UGT1A1s were transiently expressed in COS-1 cells, and the kinetic parameters of these UGT1A1s were determined for SN-38 glucuronidation. A partially reduced UGT1A1 protein expression was observed in COS-1 cells for G71R and Y486D. WT UGT1A1 catalyzed SN-38 glucuronidation with an apparent K(m) value of 11.5 microM, whereas those of G71R, P229Q, and Y486D were 14.0, 18.0, and 63.5 microM, respectively. The SN-38 glucuronidation efficiency ratio (V(max)/K(m)) normalized for the level of expression was 1.4, 0.66 (47% of WT), 0.73 (52%), and 0.07 (5%) microl/min/mg of protein for WT, G71R, P229Q, and Y486D, respectively. Thus, the SN-38 glucuronidation activity of Y486D was drastically reduced, whereas the reduction in the G71R and P229Q activities was fractional. The decreased SN-38 glucuronidation efficiency ratio of G71R and P229Q could be critical in combination with other polymorphisms in the UGT1A1 gene.     

Antibody conjugates of 7-ethyl-10-hydroxycamptothecin (SN-38) for targeted cancer chemotherapy

CPT-11 is a clinically used cancer drug, and it is a prodrug of the potent topoisomerase I inhibitor, SN-38 (7-ethyl-10-hydroxycamptothecin). To bypass the need for the in vivo conversion of CPT-11 and increase the therapeutic index, bifunctional derivatives of SN-38 were prepared for use in antibody-based targeted therapy of cancer. The general synthetic scheme incorporated an acetylene-azide click cycloaddition step in the design, a short polyethylene glycol spacer for aqueous solubility, and a maleimide group for conjugation. Conjugates of a humanized anti-CEACAM5 monoclonal antibody, hMN-14, prepared using these SN-38 derivatives were evaluated in vitro for stability in buffer and human serum and for antigen-binding and cytotoxicity in a human colon adenocarcinoma cell line. Conjugates of hMN-14 and SN-38 derivatives 16 and 17 were found promising for further development.     

中心复合效应面设计法优化7-乙基-10-羟基喜树碱(SN-38)前体脂质体处方

目的通过中心复合效应面设计法筛选最佳处方,制备7-乙基-10-羟基喜树碱(SN-38)前体脂质体。方法分别以药脂质量比(md∶ml)、磷脂质量分数(w)、Tween80质量浓度(ρ)为实验考察对象,以包封率(Y1∶wEE)、粒径(Y2∶d)为效应,利用中心复合效应面设计法筛选SN-38前体脂质体的最佳处方。结果前体脂质体的包封率为88.43%,粒径为239.00 nm,与模型预测值相比偏差均小于10%。结论使用中心复合效应面设计法可以很好的优化SN-38前体脂质体的处方。     

Development of a method to quantify total and free irinotecan and 7-ethyl-10-hydroxycamptothecin (SN-38) for pharmacokinetic and bio-distribution studies after administration of irinotecan liposomal formulation

In 2015, liposomal formulation of irinotecan (ONIVYDE) has been approved by FDA and widely applied in the treatment of pancreatic cancer. ONIVYDE is a novel liposome formulation, entrapping CPT-11 in the aqueous core of vesicles using a modified gradient loading method. Due to toxicity concerns, it is essential to explore a rapid and reliable method to effectively isolate and quantify the non-liposomal, namely, free CPT-11and total CPT-11 in plasma. This study focuses on separation of non-liposomal CPT-11, evaluation of the pharmacokinetics of free CPT-11 and total CPT-11 and bio-distribution after intravenous administration of CPT-11 liposome. Free CPT-11 in plasma was separated by solid-phase extraction (SPE). The amount of total CPT-11 and main metabolite 7-ethyl-10-hydroxycamptothecin (SN-38) in plasma was quantified by ultra-performance liquid chromatography–MS/MS. The calibration curves fitted well and lower limit of quantitation for SN-38, free CPT-11, total CPT-11 and CPT-11 in tissue and were 5 ng/ml, 10 ng/ml, 4.44 ng/ml and 25 ng/ml respectively. The recoveries, precision and accuracy of the method appear satisfactory. Using this method, the pharmacokinetics and bio-distribution of CPT-11 liposome formulation after an intravenous dose of 2.5 mg/kg were then investigated.     

Overcoming S-phase checkpoint-mediated resistance: sequence-dependent synergy of gemcitabine and 7-ethyl-10-hydroxycamptothecin (SN-38) in human carcinoma cell lines

Although agents that inhibit DNA synthesis are widely used in the treatment of cancer, the optimal method for combining such agents and the mechanism of their synergy is poorly understood. The present study examined the effects of combining gemcitabine (2',2'-difluoro 2'-deoxycytidine) and 7-ethyl-10-hydroxycamptothecin (SN-38; the active metabolite of irinotecan), two S-phaseselective agents that individually have broad antitumor activity, in human cancer cells in vitro. Colony-forming assays revealed that simultaneous treatment of Ovcar-5 ovarian cancer cells or BxPC-3 pancreatic cancer cells with gemcitabine and SN-38 resulted in antagonistic effects. In contrast, sequential treatment with these two agents in either order resulted in synergistic anti-proliferative effects, although the mechanism of synergy varied with the sequence. In particular, SN-38 arrested cells in S phase, enhanced the accumulation of gemcitabine metabolites, and diminished checkpoint kinase 1, thereby sensitizing cells in the SN-38 --> gemcitabine sequence. Gemcitabine treatment followed by removal allowed prolonged progression through S phase, contributing to synergy of the gemcitabine --> SN-38 sequence. These results collectively suggest that S-phase-selective agents might exhibit more cytotoxicity when administered sequentially rather than simultaneously.     

Development of ELISAs for irinotecan and its active metabolite SN-38

Two highly sensitive and specific enzyme-linked immunosorbent assays (ELISAs) for the determination of 7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxy-camptothecin (irinotecan) and 7-ethyl-10-hydroxycamptothecin (SN-38), an active metabolite of irinotecan, were developed, which are capable of measuring as low as 16 and 160 pg of each drug/ml, respectively. Anti-irinotecan antibody was obtained by immunizing rabbits with irinotecan conjugated with mercaptosuccinyl bovine serum albumin (MS.BSA) using N-(4-diazophenyl)maleimide (DPM) as a heterobifunctional coupling agent. An enzyme marker was similarly prepared by coupling irinotecan with horseradish peroxidase (HRP) via DPM. This ELISA for irinotecan was specific for irinotecan and showed almost no cross-reactivity with its active metabolite SN-38. Anti-SN-38 antibody was obtained by immunizing rabbits with SN-38 conjugated with BSA using the N-succinimidyl ester method. An enzyme marker was prepared by coupling SN-38 with HRP employing DPM. The ELISA for SN-38 was specific to SN-38 and showed a very slight cross-reactivity with irinotecan (0.08%). Using the 2 assays, we reconfirmed the rapid metabolite of irinotecan with rat serum. The 2 ELISAs may be a valuable tool for studies of the pharmacokinetics and pharmacodynamics of these drugs.     

A simple and sensitive LC/MS/MS assay for 7-ethyl-10-hydroxycamptothecin (SN-38) in mouse plasma and tissues: application to pharmacokinetic study of liposome entrapped SN-38 (LE-SN38)

An LC/MS/MS method to quantify SN-38 in mouse plasma and tissue homogenates containing liposome entrapped SN-38 (LE-SN38) was developed. Camptothecin (CPT) was used as the internal standard (IS). Sample preparation consisted of simple protein precipitation by acetonitrile containing 0.5% acetic acid. SN-38 and IS were separated by a C18 HPLC column and detected using a mass spectrometer operating in the multiple reaction monitoring (MRM) mode. The peak area of the m/z 393.3-->349.1 transition of SN-38 and that of the m/z 349.1-->305.2 transition of the IS were measured and a standard curve was generated from their ratios. The method had a LLOQ of 0.5 ng/mL in mouse plasma, which corresponds to 2.5 pg for the 5 microL injection volume. The linear range was 0.5-1000 ng/mL of SN-38 in plasma sample spiked with LE-SN38. The LLOQ in tissue homogenates (5%, w/v) quantitation was 1 ng/mL (20 ng/g tissue) of SN-38 in kidney, liver, lung, and spleen homogenates, and 2 ng/mL (40 ng/g tissue) in heart homogenate containing LE-SN38. The assay was linear up to 400 ng/mL of SN-38 in tissue homogenates, and may be extended to 120 microg/mL by proper dilution of samples over the upper limit of quantitation. Acceptable precision and accuracy were obtained for concentrations over the entire standard curve range, both between-run and within-run for plasma and tissue homogenates. The method was successfully used to quantify SN-38 in plasma and tissues samples for pharmacokinetic and tissue distribution studies of LE-SN38 in mice.     

Role of organic anion transporter OATP1B1 (OATP-C) in hepatic uptake of irinotecan and its active metabolite, 7-ethyl-10-hydroxycamptothecin: in vitro evidence and effect of single nucleotide polymorphisms.

Irinotecan hydrochloride (CPT-11) is a potent anticancer drug that is converted to its active metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), and other metabolites in liver. The disposition and gastrointestinal toxicity of irinotecan exhibit a wide interpatient variability. Here, we examined the contribution of an organic anion-transporting polypeptide, OATP1B1 (OATP-C), which transports a variety of drugs and their metabolites from blood to liver in humans, to the hepatic disposition of irinotecan, SN-38, and its glucuronide conjugate (SN-38G) by using HEK293 cells stably transfected with SLCO1B1*1a (OATP-C*1a) coding wild-type OATP1B1. We further examined the effect of single nucleotide polymorphisms in OATP1B1 by measuring uptake activity in Xenopus oocytes expressing OATP1B1*1a and three common variants. In all cases, transport activity for SN-38 was observed, whereas irinotecan and SN-38G were not transported. Moreover, SN-38 exhibited a significant inhibitory effect on OATP1B1-mediated uptake of [(3)H]estrone-3-sulfate. Among the variants examined, OATP1B1*15 (N130D and V174A; reported allele frequency 10-15%) exhibited decreased transport activities for SN-38 as well as pravastatin, estrone-3-sulfate, and estradiol-17beta-glucuronide. This study is the first to yield evidence that OATP1B1 is involved in the hepatic disposition of SN-38 and that genetic polymorphisms of OATP1B1 may contribute to the known interpatient variability in disposition of irinotecan.     

高效液相色谱法测定大鼠血浆中伊立替康及其活性代谢产物SN-38的含量

目的:建立同时测定大鼠血浆中伊立替康(CPT-11)及其活性代谢产物7-乙基-10-羟基喜树碱(SN-38)浓度的高效液相色谱法。方法:以10-羟基喜树碱作为内标,先用7%高氯酸酸化血浆,再用7%高氯酸-乙腈(50∶50)沉淀蛋白。采用Hypersil C18色谱柱(4.6 mm×250 mm,5μm)进行分离;以0.05 mol·L-1的磷酸氢二钠-甲醇-三乙胺(50∶50∶0.025,磷酸调pH 3.0)为流动相;荧光检测波长:激发波长380 nm,发射波长550 nm。结果:大鼠血浆中CPT-11和SN-38线性范围分别是20～5000 ng·mL-1(r=0.9997)和2～500 ng·mL-1(r=0.9999)。两组分最低检出限分别为15 ng·mL-1和1.7 ng·mL-1。2组分平均相对回收率分别是98.7%和99.9%;平均绝对回收率分别87.2%和94.7%。2组分日内、日间精密度均小于12%。结论:本方法快速、简便、准确,灵敏度高,可用于CPT-11及其活性代谢产物SN-38药代动力学的研究。     

High-performance liquid chromatographic analysis of the anticancer drug irinotecan (CPT-11) and its active metabolite SN-38 in human plasma

A rapid, sensitive, and specific high-performance liquid chromatography (HPLC) method for the simultaneous determination of irinotecan (CPT-11) and its active metabolite SN-38 in human plasma is described. The analytes are quantified as the totals of their carboxylate and lactone form. The sample pretreatment consisted of a simple protein precipitation with acetonitrile-methanol (1:1, v/v), after which CPT-11 and SN-38 were quantitatively converted to their carboxylate form by adding 0.01 mol/L sodium tetraborate (pH, 9). Chromatography was carried out on a Zorbax SB-C18 column with fluorescence detection. The method has been validated, and stability tests under various clinically relevant conditions have been performed. The lower limit of quantification (LLOQ) was 5.0 ng/mL for CPT-11 and 0.5 ng/mL for SN-38. Standard concentration ranges were linear between 5 and 1,500 ng/mL for CPT-11 and between 0.5 and 100 ng/mL for SN-38. This assay is simple, rapid, and very useful for therapeutic monitoring of CPT-11 and SN-38.     

人血浆中伊立替康及其活性代谢物7-乙基-10羟基喜树碱浓度高效液相色谱-荧光测定方法的建立

目的:建立人血浆中伊立替康(CPT-11)及其代谢物7-乙基-10羟基喜树碱(SN-38)的浓度测定方法并进行方法学考证。方法:用Luna 5u CN100A(4.6 mm×150 mm,5μm)色谱柱,乙腈与醋酸铵缓冲溶液(50 mmol.L-1,pH4)为流动相梯度洗脱,CPT-11的检测波长为Ex/Em=368 nm/432 nm,SN-38的检测波长为Ex/Em=368 nm/535 nm。结果:CPT-11保留时间为(9.3±0.3)min,SN-38保留时间为(4.8±0.3)min。空白样品在CPT-11、SN-38及内标喜树碱出峰位置均无干扰。CPT-11在46.9～6 000.0nmol.L-1的范围内线性良好,SN-38在2.0～250.0nmol.L-1的范围内线性良好,r值均为0.998。低浓度点RSD均在20%内,其余浓度点的RSD均在15%内,准确度均在85%～115%之间。血浆样品长期冻存稳定性良好,反复冻融3次及提取后室温放置24 h条件下,样品浓度均无显著变化。结论:使用高效液相色谱-荧光检测方法简便,准确,灵敏,适用于伊立替康及其活性代谢物SN-38的血药浓度检测。     

Enhanced 7-ethyl-10-hydroxycamptothecin (SN-38) lethality by methylselenocysteine is associated with Chk2 phosphorylation at threonine-68 and down-regulation of Cdc6 expression

Methylselenocysteine (MSC) is an organic selenium compound in preventative clinical trials involving prostate, lung, and colon carcinoma. We found that methioninase-activated MSC potentiates 7-ethyl-10-hydroxycamptothecin (SN-38)-induced cell lethality in vitro in the p53-defective human head and neck carcinoma A253 cells. Activated MSC increases chk2 phosphorylation at threonine-68 induced by SN-38, with no significant effect on chk1 phosphorylation. Cell cycle arrest induced by SN-38, however, was not abrogated or potentiated by MSC. These results suggest that the enhanced cellular lethality of SN-38 by MSC was not associated with cell cycle regulation pathways. Because chk2, in addition to its role in cell cycle arrest, can induce apoptosis by phosphorylation/activation, we examined whether increased chk2 phosphorylation could induce preapoptotic DNA fragmentation. DNA damage analysis showed that megabase DNA fragmentation is decreased, accompanied by the increased 30 to 300 kilobase pairs of DNA fragmentation after exposure to SN-38 with MSC, compared with SN-38 alone. No significant changes in the amount of DNA fragments were observed in cells treated with SN-38 or MSC alone. Moreover, proteolytic destruction of DNA replication-associated proteins cdc6, MCM2, and cdc25A may induce a DNA damage checkpoint response. The observed down-regulation of DNA replication proteins cdc6, MCM2, and cdc25A after exposure to SN-38 with MSC further indicates a relationship between drug response and DNA damage. Exposure to SN-38 with MSC resulted in a significant increase of poly(ADP-ribose) polymerasecleavage and caspase 3 activation. All together, the data support the hypothesis that enhanced lethality of this combination is associated with increased chk2 phosphorylation at Thr68 and down-regulation of specific DNA replication-associated proteins, which result in poly(ADP-ribose) polymerase cleavage, caspase 3 activation, and the induction of 30 to 300 kilobase pairs of DNA fragmentation.     

Involvement of up-regulation of hepatic breast cancer resistance protein in decreased plasma concentration of 7-ethyl-10-hydroxycamptothecin (SN-38) by coadministration of S-1 in rats.

The safety and efficacy of combination therapy with 7-ethyl-10-[4-[1-piperidino]-1-piperidino]carbonyloxycamptothecin (CPT-11, irinotecan) and S-1 composed of tegafur, a prodrug of 5-fluorouracil, gimeracil, and potassium oxonate, have been confirmed in patients with colorectal cancer. Previously, we showed that p.o. coadministration of S-1 decreased the plasma concentration of both CPT-11 and its metabolites in a patient with advanced colorectal cancer. The aim of this study was to clarify the mechanism of drug interaction using both in vivo and in vitro methods. Rats were administered S-1 p.o. (10 mg/kg) once a day for 7 consecutive days. On day 7, CPT-11 (10 mg/kg) was administered by i.v. injection. Coadministration of S-1 affected the pharmacokinetic behavior of CPT-11 and its metabolites, with a decrease in the C(max) and area under the plasma concentration curve (AUC) of the active metabolite 7-ethyl-10-hydroxycampothecin (SN-38) lactone form. Furthermore, the rate of biliary excretion of the SN-38 carboxylate form increased on coadministration of S-1. In the liver, the level of breast cancer resistance protein (BCRP), but not P-glycoprotein and multidrug resistance-associated protein 2, was elevated after administration of S-1. Enzymatic conversion of CPT-11 to SN-38 by carboxylesterase (CES) was unaffected by the liver microsomes of rats treated with S-1. In addition, components of S-1 did not inhibit the hydrolysis of p-nitrophenylacetate, a substrate of CES, in the S9 fraction of HepG2 cells. Therefore, the mechanism of drug interaction between CPT-11 and S-1 appears to involve up-regulation of BCRP in the liver, resulting in an increased rate of biliary excretion of SN-38 accompanied by a decrease in the C(max) and AUC of SN-38.     

The transformation of irinotecan (CPT-11) to its active metabolite SN-38 by human liver microsomes Differential hydrolysis for the lactone and carboxylate forms

Irinotecan (CPT-11) is a new camptothecine derivative presently in development for the treatment of several advanced malignancies. It is converted in vivo to a highly potent metabolite, SN-38, by carboxylesterases. All camptothecine derivatives undergo lactonolysis in a pH-dependent reversible manner, generating inactive carboxylate forms. We have investigated in vitro the kinetics of transformation of CPT-11 to SN-38 by human liver microsomes originating from several donors. Microsomes from seven livers were studied individually or as a pooled preparation. CPT-11, either in its lactone or its carboxylate form, was added at a range of concentrations. The SN-38 formed was measured by HPLC with fluorometric detection. In the deacylation-limited carboxylesterase reaction, the linear steady-state kinetics between 10 and 60min were determined. At all concentrations of CPT-11, the steady-state velocity of SN-38 formation as well as the intercept concentrations of SN-38 were about 2-fold higher when the substrate was under the lactone form than under the carboxylate form. We estimated the values (±SD) of K’_m and V _max to be 23.3±5.3μM and 1.43±0.15pmol/min/mg for the lactone and 48.9±5.5μM and 1.09±0.06pmol/min/mg for the carboxylate form of CPT-11, respectively. We conclude that the greater rate of conversion of CPT-11 lactone may contribute to the plasma predominance of SN-38 lactone observed in vivo. The inter-individual variation of SN-38 formation was relatively high (ratio of 4 between extreme values) but no large age- or gender-related differences were seen. The effect of twelve drugs of different therapeutic classes (antibiotics, antiemetics, antineoplastics, antidiarrhoeics, analgesics), which could be administered in association with irinotecan in the clinical setting, was evaluated in this system (drug concentration: 100μM; CPT-11 lactone concentration: 10μM). Loperamide and ciprofloxacine where the only drugs exerting a weak inhibition of CPT-11 conversion to SN-38. Received: 30 December 1996 /Accepted: 30 April 1997     

UDP-葡萄糖醛酸转移酶1A1基因多态性及其与伊立替康化疗不良反应的关系

目的 研究中国进展期胃肠道肿瘤患者UDP-葡萄糖醛酸转移酶1A1（UGT1A1）的基因多态性及其与伊立替康化疗不良反应的发生率和严重程度的关系.方法 利用聚合酶链反应-连接酶检测反应（PCR-LDR）等方法对202例进展期胃肠道肿瘤患者进行UGT1A1基因多态性检测.所有患者均采用含伊立替康方案化疗,观察并记录化疗中出现的不良反应情况,比较不同基因型患者使用伊立替康后不良反应发生率的差异.结果 202例患者中UGT1A1野生型TA6/TA6 156例（77.2％）;杂合突变型TA6/TA744例（21.8％）,纯合突变型TA7/TA7 2例（1.0％）.3～4度不良反应的情况：腹泻27例（13.4％）、白细胞减少19例（9.4％）,TA6/TA6与TA6/TA7基因型患者出现3级以上腹泻、白细胞减少与1～2级之间差异无统计学意义（P＞0.05）.TA7/TA7基因型患者出现3级以上腹泻100％,白细胞减少50％.结论 用伊立替康化疗前行UGT1A1基因多态性检测可以筛查高危人群,预测伊立替康的严重不良反应,以指导临床用药.     

Differential rates of glucuronidation for 7-ethyl-10-hydroxy-camptothecin (SN-38) lactone and carboxylate in human and rat microsomes and recombinant UDP-glucuronosyltransferase isoforms.

7-ethyl-10-hydroxy-camptothecin (SN-38), the active metabolite of the anti-cancer agent irinotecan, contains a lactone ring that equilibrates with a carboxylate form. Since SN-38 lactone is the active and toxic form, it is prudent to examine whether the more soluble carboxylate is a surrogate for SN-38 lactone conjugation. Therefore, relative rates of glucuronidation and isoform specificity of SN-38 lactone and carboxylate were characterized. The stability of SN-38 lactone and carboxylate in incubation mixtures of microsomes and UDP-glucuronosyltransferase (UGT) isoforms was used to determine optimal incubation times. Microsomal incubations were conducted using rat and human intestinal and hepatic microsomes and human and rat recombinant UGT1A isoforms. Where estimates of lactone and carboxylate glucuronidation rates could not be established due to short incubation times and detection limits, kinetic modeling was used to recover these rate constants. The stability experiments revealed that the lactone was stabilized by rat microsomes, however, the opposite was observed in human microsomes and recombinant isoforms. For all tissues and most UGT isoforms examined, the lactone consistently had catalytic rates up to 6-fold greater than the carboxylate. The rank order of glucuronidation for both SN-38 lactone and carboxylate was 1A7 > 1A1 > 1A9 > 1A8 and 1A7 > 1A8 > 1A1 for human and rat isoforms, respectively. This study provides further support that SN-38 lactone and carboxylate may be considered pharmacokinetically distinct agents. The in vivo impact of this conjugation difference is unknown, since variations in protein binding and transport proteins may affect intracellular concentrations of the lactone or carboxylate.     

Influence of genetic variants in UGT1A1 and UGT1A9 on the in vivo glucuronidation of SN-38

The uridine diphosphate glucuronosyltransferase (UGT) 1A1 and 1A9 isoforms are involved in the phase II biotransformation of the irinotecan metabolite, SN-38. Recently, several variants in the UGT1A1 and UGT1A9 genes have been described with altered functionality in vitro. The aim of this study was to evaluate the functional consequence of the UGT1A1(TA)(7)TAA (UGT1A1(*)28), UGT1A9 766G>A (D256N; UGT1A9(*)5), and UGT1A9 98T>C (M33T; UGT1A9(*)3) variants in Caucasian patients treated with irinotecan. Pharmacokinetic studies were performed after the first course of irinotecan in 47 males and 47 females. The mean (SD) area under the curves (AUCs) of irinotecan and SN-38 were 20,348 +/- 6466 ng x h/mL and 629 +/- 370 ng x h/mL, respectively, which is in line with earlier findings. For UGT1A9(*)5,novariant alleles were observed, whereas for UGT1A9(*)3, 1 patient with the variant allele was found (allele frequency, 0.633%). The distribution of the UGT1A1(*)28 variant showed 44 wild-type patients (Wt), 37 heterozygotes (Het), and 5 homozygotes (Var). The median AUC ratio of SN-38G to SN-38 was significantly reduced in carriers of the variant UGT1A1(*)28 allele (7.00 [Wt] vs. 6.26 [Het] vs. 2.51 [Var]; p =.022). It is concluded that UGT1A9 functional variants are rare in Caucasians and likely to be clinically insignificant in irinotecan regimens. Screening for the UGT1A1(*)28 polymorphism may identify patients with altered SN-38 pharmacokinetics.     

The novel UGT1A9 intronic I399 polymorphism appears as a predictor of 7-ethyl-10-hydroxycamptothecin glucuronidation levels in the liver.

Polymorphisms in UGT1A9 were associated with reduced toxicity and increased response to irinotecan in cancer patients. UDP-glucuronosyltransferase (UGT) protein expression, glucuronidation activities for 7-ethyl-10-hydroxycamptothecin (SN-38), and probe substrates of the UGT1A9 and UGT1A1 were measured in 48 human livers to clarify the role of UGT1A9 variants on the in vitro glucuronidation of SN-38. Genotypes were assessed for UGT1A9 (-2152C>T, -275T>A, and -118T(9>10)), three novel UGT1A9 variants (-5366G>T, -4549T>C, and I399C>T), and UGT1A1 (-53TA(6>7), -3156G>A, and -3279T>G). Of all the variants, the UGT1A9 I399C>T was associated with the most dramatic change in SN-38-glucuronide (SN-38G) (2.64-fold; p = 0.0007). Compared with UGT1A9 I399C/C, homozygous I399T/T presented elevated UGT1A1 and UGT1A9 proteins and higher glucuronidation of UGT1A9 and UGT1A1 substrates (p < 0.05). The very low linkage disequilibrium (r(2) < 0.19) between UGT1A9 I399 and all the other UGT1A1 and UGT1A9 variants suggests a direct effect or linkage to unknown functional variant(s) relevant to SN-38 glucuronidation. The UGT1A9 -118T(9/10) was also linked to alteration of SN-38 glucuronidation profiles in the liver (p < 0.05) and was associated with higher UGT1A1 protein (p = 0.03). However, UGT1A9 -118T(10) appears to have low functional impact as a result of the lack of correlation with UGT1A9 protein levels and a modest 1.4-fold higher reporter gene expression associated with the -118T(10) allele in HepG2 cells (p = 0.004). In contrast, the UGT1A9 -5366T, -4549C, -2152T, and -275A, associated with higher UGT1A9 protein (2-fold; p < 0.05), have no influence on SN-38G. Despite limitations resulting from sample size, results indicate that UGT1A9 I399 and -118T(9/10) may represent additional candidates in combination with UGT1A1 promoter haplotypes for the prediction of SN-38 glucuronidation profile in vivo.     

7-乙基-10-羟基喜树碱脂质体在大鼠体内的代谢和排泄

目的考察7-乙基-10-羟基喜树碱(SN-38)脂质体经静脉注射后,在大鼠尿液、粪便中的代谢产物以及以SN-38原形药物排泄的量。方法大鼠尾静脉单次给予2.77 mg/kg SN-38脂质体,分别于0～6、6～12、12～24、24～48 h分段收集尿液、粪便,采用UPLC/Q-TOFMS法对SN-38脂质体在大鼠尿液、粪便中的代谢产物进行鉴定,并且建立HPLC法,用于大鼠尿液及粪便样品中SN-38原形药物的排泄量的测定。结果 SN-38脂质体的在大鼠体内的代谢产物经鉴定为SN-38G。48 h内脂质体组共有1.57%的原形药物经过尿液排出,共有12.94%的SN-38原形药物经过粪便排出。结论 SN-38脂质体只有少部分以原形药物经尿液和粪便排出体外。