Evaluation of the Japanese opioid conversion ratio for switching to methadone for cancer pain control.

196 Background: In Japan, oral administration of methadone was approved for patients with cancer pain in March 2013. As methadone exhibits complex pharmacokinetics with individual differences and rare but serious adverse effects, methadone is only prescribed as a fourth-line drug by cancer pain specialists, who must start methadone according to the following table and must not increase methadone dose within 7 days.

AIMS: To assess the validity of the Japanese opioid conversion ratio.

METHODS: The clinical characteristics of 60 patients who were prescribed oral methadone between April 2013 and March 2016 were analyzed.

RESULTS: The switch from other opioids to methadone was initiated due to refractory pain in the stop-and-go switching. According to the table in Japan, the starting dose of methadone ranged from 15-45mg/day, depending on the previous opioid dose. Fifty cases (11 outpatients, 39 inpatients) were successfully switched to methadone; although 10 cases subsequently exhibited rapid progression of illness and failed due to oral difficulty during the course of dose titration. At the outset, the average oral morphine equivalent daily dose before methadone administration was 155mg (range, 40-660mg) and the starting methadone dose was 10 mg in 2 cases (extremely old age and multi-drug taking), 15 mg in 35 cases, 30 mg in 11 cases and 45 mg in 2 cases. Upon completion of the dose titration according to the Japanese definition, the methadone dose was the same as the starting dose in 21 cases, and was decreased or increased from the starting dose in 5 and 24 cases, respectively.

CONCLUSIONS: TheJapanese opioid conversion ratio might be better corrected in the near future. For example, it is good to be able to start with 10mg or 20 mg because minute changes might lead the performance of low dose titration in some cases. It should be possible to increase the dose of methadone after 3 or 4 days from the later change based upon the pain severity. [Table: see text].