Causative agent of vascular pain among photodegradation products of dacarbazine

The photodegradation products of the anticancer drug, dacarbazine, cause adverse reactions including local venous pain when injected intravenously. In this study, we attempted to identify which of these products is responsible. We synthesized or purchased five photodegradation products of dacarbazine (dimethylamine, 5-diazoimidazole-4-carboxamide (Diazo-IC), 4-carbamoylimidazolium-5-olate, 5-carbamoyl-2-(4-carbamoylimidazol-5-ylazo)imidazolium-5-olate and 2-azahypoxanthine) and examined the pain reaction induced by their intraperitoneal administration in mice using an abdominal stretching or constriction assay. Only Diazo-IC clearly induced pain reaction in mice in a dose-dependent manner, the other products caused no pain reaction. The threshold concentration for pain reaction in mice was estimated to be about 0.1 mg mL-1. While diclofenac sodium significantly reduced acetic-acid-induced pain reaction in mice, it did not influence those induced by Diazo-IC. This result suggests that the mechanism of Diazo-IC-induced pain is different from that of acetic-acid-induced inflammatory pain. Dacarbazine itself produced marked relaxation of rat thoracic aorta strips in a concentration-dependent manner, but there was no difference between the activity of dacarbazine and its photo-exposed solution, so constriction or relaxation of blood vessels is unlikely to be a factor in the pain reaction. In conclusion, Diazo-IC generated by photodegradation of dacarbazine solution causes the side-effect of venous pain. Dacarbazine solution that has turned pink should not be used, because Diazo-IC is an intermediate in the formation of the reddish product, 5-carbamoyl-2-(4-carbamoylimidazol-5-ylazo)imidazolium-5-olate. Drip infusion preparations of dacarbazine should be shielded from light.