[Assessment of total bilirubin or SN-38/SN-38G ratio as a predictor of severe irinotecan toxicity].

SN-38, an active metabolite of irinotecan (CPT-11), is glucuronidated into SN-38G mainly by UGT1A1, during detoxification. However, significant interindividual pharmacokinetic variability in SN-38 is caused by factors, including inherited predispositions that may affect the function and expression of UGT1A1. Moreover, these individual differences can contribute to the development of clinical conditions, such as severe leucopenia and diarrhea. Similar to SN-38, bilirubin is excreted into bile after being glucuronidated by UGT1A1. Thus, bilirubin is metabolized by a mechanism similar to that of SN-38. This suggests that the bilirubin level may be an indicator of the adverse effects caused by CPT- 11. On the other hand, the ratio between the AUC of SN-38 and the AUC of SN-38G (AUCSN-38/AUCSN-38G) indicates the ability of SN-38 to be glucuronidated, and is known to correlate with leucopenia and diarrhea. However, many blood sampling points are required to calculate these AUCs. Therefore, the daily estimation of the AUCSN-38/AUCSN-38G values of individual patients is not practical at the clinical level. Thus, the objectives of this study were as follows: (1) to establish whether or not the total bilirubin level is a useful indicator in predicting the development of CPT-11 toxicity. (2) to investigate the correlation of SN-38/SN-38G (the ratio of the serum concentrations of SN-38 and SN-38G) with AUCSN-38/AUCSN-38G. Based on the result of this investigation, it will be discussed whether or not SN-38/SN-38G may be used as an alternative to AUCSN-38/AUCSN-38G. This study included 14 patients with small cell lung cancer or non-small-cell lung cancer, in whom serum concentrations of CPT-11, SN-38, and SN-38G were measured by HPLC. The results demonstrated a significant correlation between the total bilirubin levels prior to chemotherapy and the logarithmic values for AUCSN-38/AUCSN-38G (r2=0.852). Among the cases with high values for both the total bilirubin level and the AUCSN-38/AUCSN-38G ratio, none of the patients had grade-3 diarrhea, while many cases tended to have grade-3 to -4 neutropenia. Additionally, the results of regression analysis suggest that SN-38/SN-38G (2 hr) and SN-38/SN-38G (4 hr) might be preferable as a predictive index for AUCSN-38/AUCSN-38G. These findings suggest that the total bilirubin level and SN-38/SN-38G, 2 to 4 hours after administration might be used as indicators to predict CPT-11-induced neutropenia. These indicators are likely to contribute to the pharmacogenetic analysis of UGT1A1 genes, as well as individualized therapy, in future, and further studies on this subject are expected.