Protection of mice from lethal doses of methotrexate by transplantation with transgenic marrow expressing drug-resistant dihydrofolate reductase activity

Marrow cells from previously established lines of transgenic mice expressing either of two different methotrexate (MTX)-resistant dihydrofolate reductases (DHFRs) were transplanted into recipient animals to determine the resultant in vivo protective effect against toxicity associated with MTX administration. Sublethally irradiated, untransplanted animals were first used to establish conditions of low-dose MTX administration resulting in substantial hematopoietic toxicity with undetectable gastrointestinal toxicity. Under these conditions, low survival rates were observed for normal or transgenic animals not expressing drug-resistant DHFR activity, whereas transgenic animals expressing either the arg22 (line 04) or trp31 (line 03) DHFR variants survived. Transplantation of 10⁶ marrow cells from either transgenic lines 03 or 04 rescued normal FVB/N recipient animals from low- dose MTX administration, which was lethal for animals transplanted with 10⁶ normal FVB/N marrow cells. Reduced survival of transgenic line 04 marrow recipients was observed when twofold or fourfold doses of MTX were administered. However, when 10⁷ transgenic line 04 marrow cells were infused, the recipients were found to be resistant to a MTX dose that was not only lethal for animals transplanted with 10⁷ normal FVB/N marrow cells, but also lethal for normal, untransplanted FVB/N mice. Histologic analysis showed protection of both marrow and gastrointestinal tissues from MTX toxicity in transgenic line 04 marrow transplant recipients. Thus, exclusive expression of MTX-resistant DHFR activity in the marrow had a substantial, systemic chemoprotective effect in animals, which could be applied for improved utilization of MTX for antitumor chemotherapy.