The effect of methotrexate on DNA synthesis and its reversal by folinic acid.

Transient inhibition of DNA synthesis has been used as an experimental technique in a number of situations. Massive doses of methotrexate, a folic acid antagonist, have been used to inhibit humoral (Berenbaum & Brown, 1965) and cellular immune responses (Berry, 1969) and to delay the appearance of vertebral ossification centres (Berry, 1971). More recently its effects on the expression of a polygenically determined malformation have been described (Berry & Germain, 1972). Although thymidine deficiency has been demonstrated in animals exposed to methotrexate for 8 h (see Berry and Germain) there has been no direct demonstration of the effects of the drug on DNA synthesis in the tissues of rapidly growing animals, nor of the efficiency of the specific antagonist, folinic acid. DNA is synthesized from purine and pyrimidine nucleotides. Methotrexate acts by inhibiting the enzyme dihydrofolate reductase, preventing the formation of tetrahydrofolate, the co-enzyme (as 5,10-methylene tetrahydrofolate) in the conversion of deoxyuridate (d-UMP) to thymidylate (d-TMP). Folinic acid counteracts this effect by supplying the product of the blocked reaction. We have measured the rate of incorporation of deoxyuridine into rapidly growing mouse liver, shortly after birth. The inhibitory effect of methotrexate and its prevention by folinic acid are reported, together with the results of simple histochemical studies designed to assess the effects of therapy on certain cellular activities.