The N-(phosphonacetyl)-L-aspartic acid (PALA)-resistant variant of the Lewis lung carcinoma, LL/PALA-C, has a target enzyme (L-aspartate transcarbamylase) activity identical to that of its parent PALA-sensitive counterpart; however, the activity of the antecedent enzyme of de novo pyrimidine biosynthesis, carbamyl phosphate synthetase II (CPS II), is elevated two-fold in this resistant variant. Inasmuch as inhibition of L-aspartate transcarbamylase by PALA is competitive with respect to the substrate carbamyl phosphate, investigations were conducted to determine the effects of carbamyl phosphate depletion, provoked by inhibition of CPS II, on the biochemical and therapeutic activity of PALA in this tumor line. Towards this end, a spectrum of L-glutamine antagonists were screened in vitro as possible inhibitors of crude extracts of CPS II (a Lglutamine- utilizing amidotransferase) prepared from LL/PALAC tumors. Acivicin was found to be the best of the CPS II inhibitors. The kinetics of Acivicin inhibition of CPS II activity was established. Doses greater than 10 mg Acivicin per kg inhibit CPS II activity in LL/PALA-C tumors up to 85% as assessed by direct enzyme assay and by measuring the inhi bition of orotate and orotidine accumulation in tumors of pyrazofurin- treated mice. Restitution of tumor CPS II following a single treatment with 10 mg Acivicin per kg was rapid (f,/2 = 24 hr). Estimation of the impact on pyrimidine biosynthesis of Acivicin used singly and in combination with PALA were made by measuring the inhibition of the accumulation of orotate and orotidine provoked by treatment with pyrazofurin. Combina tions of 10 to 25 mg Acivicin per kg and 200 mg PALA per kg significantly enhanced inhibition of pyrimidine biosynthesis over single-drug activities. On the basis of these and collateral studies with PALA, a therapeutic trial was set up to evaluate the combination of PALA and Acivicin against LL/PALA-C tumors. The results of this trial show that, as single agents, neither drug was active against this line; however, combina tions of 5 or 10 mg Acivicin per kg (every day, Days 1 to 9) and 200 mg PALA per kg (every other day, Days 1 to 9) showed >80% inhibition of tumor growth on Days 10 and 21. Long-term cures (3/10) and a 50% increase in life span were observed in the 10-mg Acivicin per kg plus 200-mg PALA per kg treatment group. Higher doses of Acivicin (e.g., 25 mg/kg) given with PALA were toxic. These experiments suggested that (a) carbamyl phosphate production is an important modulator of PALA therapeutic activity and (b) the rigorous biochemical definition of a rationally designed drug combination protocol can provide enhanced therapeutic results.
|Original language||English (US)|
|Number of pages||5|
|State||Published - Mar 1 1981|
ASJC Scopus subject areas
- Cancer Research