In vitro conversion of irinotecan to SN‐38 in human plasma

Abstract

Irinotecan is an active cytotoxic agent for various cancers, and is converted to SN-38, its most active metabolite, by carboxylesterase converting enzyme (CCE) in vivo. Although the primary metabolic site is in the liver, ex vivo studies have proven that irinotecan is also converted to SN-38 in intestines, plasma and tumor tissues. The present study attempted to elucidate the in vitro conversion efficiency in human plasma, and to examine possible inter-individual variability and its clinical significance. Plasma samples were taken from 57 patients with lung cancer, 3 patients with benign pulmonary diseases and 9 healthy volunteers. After addition of 157 µ µ µ µM irinotecan to plasma, time courses of SN-38 concentration, measured by high-performance liquid chromatography (HPLC), were investigated. All subjects showed linear increase in SN-38 concentration during the first 60-min period, followed by a plateau. Mean and standard deviation of the conversion rate in the first 60 min were 515.9 ± ± ± ±50.1 pmol/ml/h (n = = = =69), with a coefficient of variation of 0.097. Although most of the subjects showed comparable conversion rates, 3 subjects had significantly higher conversion rates. In conclusion, the results of this study suggest that the enzyme activity of CCE in human plasma may show inter-individual variability. rinotecan, a camptothecin analogue, has recently been introduced into clinical practice and used for treatment of various cancers including lung, colorectal, pancreatic and ovarian cancer. It is a pro-drug that is converted to SN-38 by carboxylesterase converting enzymes (CCE), 1) and SN-38 has 100-to 1000-fold higher cytotoxic activity when compared to the original chemical form.2, 3) In addition, irinotecan and its metabolites have an extremely complex pharmacokinetic profile that is dependent on many enzymes involved in metabolism and transporters associated with intestinal absorption and hepatobiliary secretion mechanisms. 4)The dose-limiting toxicity of the agent includes bone marrow suppression and diarrhea. These adverse effects are occasionally severe not only in high-risk patients, and can even be fatal, especially when diarrhea is accompanied by neutropenia. 5-8)One of the major clinical issues with this drug is that its toxicity and pharmacokinetic parameters ostensibly show inter-patient variability, 4,9) and the toxicity is not necessarily correlated to the pharmacokinetic parameters. 4) Therefore, predicting its toxicity in any given patient has been difficult in contrast to other agents such as carboplatin, for which a sophisticated method for individualizing dosing is available. 10)As one of the mechanisms underlying this inter-individual variability of irinotecan metabolism, Ando et al. 11,12) demonstrated that bilirubin UDP-glucuronosyltransferase 1 gene promoters with excessive TATAA elements were responsible for reduced conjugation of SN-38 to SN-38-glucuronide (SN-38G), resulting in a reduction of SN-38 detoxication in the liver. However, it is still unresolved whether this mec...

Irinotecan Toxicity to Human Blood Cells in vitro: Relationship between Various Biomarkers

“…The actual levels of DNA damage produced both in cancer and non‐target cells in human evidently are much higher due to the presence of SN‐38, a metabolite with an augmented potency and cytotoxicity [26]. While the present study was performed in vitro , and the blood cells were exposed to irinotecan in a serum‐free medium, there was no possibility for extensive conversion of the parent drug to SN‐38, although some studies indicate that human plasma esterases, especially butyrylcholinesterase, present in serum also possess irinotecan‐activating activity [51,52].…”

OATP1A/1B Transporters Affect Irinotecan and SN-38 Pharmacokinetics and Carboxylesterase Expression in Knockout and Humanized Transgenic Mice

“…Conversion of irinotecan to SN-38 can be mediated by several esterases (32), which are usually synthesized in the liver, but some also in the small intestine and kidney (29,33). Therefore, we tested expression levels by real time PCR of all obvious candidate carboxylesterase, paraoxonase, and butyrylcholinesterase genes (34) in the liver, small intestine, and kidney of wild-type and Oatp1a/1b knockout, but also of liver-specific humanized OATP1B1 and OATP1B3 transgenic mice (Fig.…”

Carboxylesterase Isoform 2 mRNA Expression in Peripheral Blood Mononuclear Cells Is a Predictive Marker of the Irinotecan to SN38 Activation Step in Colorectal Cancer Patients

“…Guemei et al (15) highlighted an association between the specific plasma esterase activity and SN38 AUC. Conversely, Shingoji et al (26) reported no association between the activity of CPT11 hydrolase in plasma (mediated by CES) and the AUC of SN38 in cancer patients.…”