The Association of Increased Dihydrofolate Reductase with Amethopterin Resistance in Mouse Leukemia* MORRISFRIEDKIN,ELIZABETHCRAWFORD,STEWARTR. HUMPHREYS, ANDABRAHAMGOLDIN

New antifolate-resi slant sublines of mouse leukemia L1210 have been developed. Dihydrofolate reductase activities were determined during early stages of increasing resistance to the folate antagonists. A daily dosage level of 0.75 mg amethopterin/kg used to develop subline FR-1 yielded tumor cells with a tenfold increase of enzyme within three serial transplants. When a new selective influence was imposed by the dichloroamethopterin treatment of mice given implants of amethopterin-resistant subline FR-1, an increase to 40 times the activity of the original parent cells occurred. When dichloroamethopterin was used alone for the development of subline FR-3 without any prior selection of resistant cells with amethopterin, partial resistance to the chloro-derivative occurred without any concomitant rise in dihydrofolate reductase. The development of full resistance to dichloroamethopterin resulted in a significant increase of dihydrofolate reductase activity.