Investigation of 6-thioguanine as a strategy to overcome methotrexate resistance in a mouse model of leptomeningeal carcinomatosis.

PURPOSE: Leptomeningeal carcinomatosis (LC), the dissemination of malignant cells into the cerebrospinal fluid, occurs in 3-5% of patients with solid tumors and is being recognized more frequently due to prolonged survival with systemic therapies. The prognosis remains dismal, with a median survival of 4-8 weeks. Methotrexate (MTX), the current standard treatment, is often compromised by resistance through dihydrofolate reductase (DHFR) upregulation and by neurotoxicity at high doses, underscoring the need for alternative therapeutic approaches.

METHODS

An MTX-resistant subline (R-MM46) was established from a murine mammary carcinoma. Resistance was confirmed by increased DHFR activity, enhanced drug efflux, and apoptosis resistance, and validated in an LC mouse model. Metabolic alterations were assessed by measuring phosphoribosyl pyrophosphate (PRPP), hypoxanthine-guanine phosphoribosyltransferase (HGPRT), and thymidine kinase (TK). The therapeutic efficacy of 6-thioguanine (6-TG), which targets the salvage pathway, was evaluated in vivo.

RESULTS

R-MM46 cells exhibited a 6-7-fold increase in DHFR activity, together with upregulation of P-glycoprotein and Bcl-2. In the LC mouse model inoculated with R-MM46 cells, MTX treatment failed to prolong survival. R-MM46 cells demonstrated PRPP accumulation and increased HGPRT and TK activity, consistent with activation of the salvage pathway. Oral 6-TG significantly extended survival, with the greatest benefit observed when administered sequentially 2-6 h after MTX.

CONCLUSION

Sequential 6-TG administration capitalizes on salvage pathway activation in MTX-resistant LC and may represent a promising therapeutic strategy to overcome MTX resistance.