Further Evidence for a Basis of Selective Activity and Relative Responsiveness during Antifolate Therapy of Murine Tumors1

A greater persistence of unbound (exchangeable) drug in tumor cells versus drug-limiting normal tissue (proliferating epithelium of small intestine) correlates with the therapeutic effects of various antifolates against a group of murine tumors. After approximate equimolar doses (3 mg/kg i.p.) of methotrexate (MTX), methasquin (MQ), aminopterin, and N|[2,4-diamino-5-chloro-6-quinazolinyl)methyl]amino)benzoyl|-L-glutamate (5-Cl-deaza-AM), total accu mulation in small intestine was fiveto eight-fold greater than the dihydrofolate reducÃ-asecontent. Free drug per sisted for less than 4 hr (MTX), 16 hr (MQ), 30 hr (ami nopterin), and 48 hr (5-Cl-deaza-AM). Overall drug accu mulation in L1210 cells was greater (12to 40-fold enzyme level), and drug persistence above enzyme level was more prolonged in the case of MTX (24 hr), MQ (32 hr), and 5-Cl-deaza-AM (greater than 48 hr). Persistence of ami nopterin was similar to that seen in small intestine. After the same dose of each drug s.c., the results were similar in small intestine. In L1210 cells, however, the total drug accumula tion was much lower, but the relative persistence of each was similar to that seen after an i.p. dose. After a single optimal therapeutic dose (3, 0.75, 0.3, and 0.1 mg/kg i.p. for MTX, MQ, aminopterin, and 5-Cldeaza-AM every other day), accumulation of each drug in small intestine varied but, in each case, free drug persisted for no more than 4 hr. In L1210 cells, maximal accumula tion of each drug also varied, but persistence differed in accordance with the relative therapeutic effectiveness of each (9 hr for 5-Cl-deaza-AM, 12 hr for aminopterin, and more than 20 hr for MTX and MQ). Following treatment with 3 mg/kg, either s.c. or i.p., free MTX persisted for 4 hr in normal liver, but it persisted for 24 hr in leukemic liver extensively infiltrated with L1210 cells. Following the administration of MTX, 3 mg/kg s.c., maximum accumulation (1 to 2 hr) in S180, Ehrlich, P288, P388, and L1210 cells approached, but did not reach, that seen in small intestine. Persistence of free MTX in tumor cells varied from 3 hr (SI80) to nearly 20 hr (L1210) and correlated with the relative therapeutic responsiveness for each tumor. Following the same dose i.p., accumulation in tumor cells was greater than in small intestine. With the exception of S180 cells, the overall level of persistence of free MTX in tumor cells was greater than for a s.c. dose but also correlated closely with therapeutic responsiveness. Data on the onset and duration of inhibition of DNA synthesis ([3H]deoxyuridine incorporation) in the various