| Abstract
| - Nineteen previously undescribed 2,4-diamino-6-(arylmethyl)-5,6,7,8-tetrahydroquinazolines(5a−m, 10−12) were synthesized as part of a larger effort to assess the therapeutic potentialof lipophilic dihydrofolate reductase (DHFR) inhibitors against opportunistic infections of AIDS.Condensation of appropriately substituted (arylmethyl)triphenylphosphoranes with 4,4-ethylenedioxycyclohexanone, followed by hydrogenation (H2/Pd−C) and acidolysis, yielded thecorresponding 4-(arylmethyl)cyclohexanones, which were then condensed with cyanoguanidineto form the tetrahydroquinazolines. Three simple 2,4-diamino-6-alkyl-5,6,7,8-tetrahydroquinazoline model compounds (9a−c) were also prepared in one step from commerciallyavailable 4-alkylcyclohexanones by this method. Enzyme inhibition assays against rat liverDHFR, Pneumocystis carinii DHFR, and the bifunctional DHFR-TS enzyme from Toxoplasmagondii were carried out, and the selectivity ratios IC50(rat)/IC50(P. carinii) and IC50(rat)/IC50(T. gondii) were compared. The three most potent inhibitors of P. carinii DHFR were the 2,5-dimethoxybenzyl (5j), 3,4-dimethoxybenzyl (5k), and 3,4,5-trimethoxybenzyl (5l) analogues,with IC50 values of 0.057, 0.10, and 0.091 μM, respectively. The remaining compounds generallyhad IC50 values in the 0.1−1.0 μM range. However all the compounds were more potent againstthe rat liver enzyme than the P. carinii enzyme and thus were nonselective. The T. gondiienzyme was always more sensitive than the P. carinii enzyme, with most of the analoguesgiving IC50 values of 0.01−0.1 μM. Moderate 5−10-fold selectivity for T. gondii versus rat liverDHFR was observed with five compounds, the best combination of potency and selectivity beingachieved with the 2-methoxybenzyl analogue 5d, which had an IC50 of 0.014 μM and a selectivityratio of 8.6. One compound (5l) was tested for antiproliferative activity against P. cariniitrophozoites in culture at a concentration of 10 μg/mL and was found to completely suppressgrowth over 7 days. The suppressive effect of 5l was the same as that of trimethoprim (10μg/mL) + sulfamethoxazole (250 μg/mL), a standard clinical combination for the treatment ofP. carinii pneumonia in AIDS patients. Four compounds (5a,h,k,l) were tested against T. gondiitachyzoites in culture and were found to have a potency (IC50 = 0.1−0.5 μM) similar to that ofpyrimethamine (IC50 = 0.69 μM), a standard clinical agent for the treatment of cerebraltoxoplasmosis in AIDS patients. Compound 5h was also active against T. gondii infection inmice when given qdx8 by peritoneal injection at doses ranging from 62.5 (initial dose) to 25mg/kg. Survival was prolonged to the same degree as with 25 mg/kg clindamycin, anotherwidely used drug against toxoplasmosis. Three compounds (5j−l) were tested for antiproliferative activity against human tumor cells in culture. Among the 25 cell lines in the NationalCancer Institute panel for which data were confirmed in two independent experiments, theIC50 for at least two of these compounds was <10 μM against 17 cell lines (68%) and in the0.1−1 μM range against 13 cell lines (52%). One compound (5j) had an IC50 of <0.01 μM againstfour of the cell lines. The activity profiles of 5k,l were generally similar to that of 5j exceptthat there were no cells against which the IC50 was <0.01 μM.
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