Antimalarial action of nitrobenzylthioinosine in combination with purine nucleoside antimetabolites.

The infection of human erythrocytes by two strains of the human malarial parasite, Plasmodium falciparum (FCQ-27 or the multi-drug-resistant strain K-1), markedly changed the transport characteristics of the nucleosides, adenosine and tubercidin, compared to uninfected erythrocytes. A component of the transport of these nucleosides was insensitive to the classical mammalian nucleoside transport inhibitor nitrobenzylthioinosine (NBMPR). In vitro studies with tubercidin demonstrated ID50 values of 0.43 and 0.51 microM for FCQ-27 and K-1, respectively. In addition, the nucleoside transport inhibitors NBMPR, nitrobenzylthioguanosine (NBTGR), dilazep and dipyridamole also independently exhibited antimalarial activity in vitro. The combination of tubercidin and NBMPR or NBTGR in vitro demonstrated synergistic activity, whilst tubercidin together with dilazep or dipyridamole showed subadditive activity. Analysis by HPLC indicated that NBMPR could permeate the infected cell membrane and provided evidence for the catabolism of NBMPR in vitro, with subsequent alteration of the purine pool in the infected erythrocyte. These observations further indicated the possibility of the utilization of cytotoxic nucleosides against P. falciparum infection in conjunction with a nucleoside transport inhibitor to protect the host tissue.

The in vitro and in vivo antimalarial activity of some Mannich bases derived from 4-[7'-bromo (and chloro)-1',5'-naphthyridin-4'-ylamino]phenol and 4-(7'-trifluoromethylquinolin-4'-ylamino)phenol.

A series of di-Mannich bases derived from 4-[7'-bromo (and chloro)-1',5'-naphthyridin-4'-ylamino]phenol and 4-(7'-trifluoromethylquinolin-4'-ylamino)phenol were assayed for activity against chloroquine-sensitive and chloroquine-resistant isolates of cultured Plasmodium falciparum using the inhibition of uptake of radiolabelled hypoxanthine. A number of the 4-(7'-trifluoromethylquinolinyl-amino)phenols showed statistically superior activity to chloroquine and amodiaquine against both isolates. Analysis of the antimalarial activity of some of these compounds against Plasmodium berghei in mice following oral administration again demonstrated activity equal or superior to that of the established antimalarials against a chloroquine-sensitive strain, and in some cases appreciably superior activity against a chloroquine-resistant strain.

Steady-state kinetics of proguanil and its active metabolite, cycloguanil, in man.

The pharmacokinetics of proguanil and its active metabolite, cycloguanil, were determined at steady-state in 6 healthy male volunteers after daily administration of 2 Paludrine tablets (200 mg proguanil hydrochloride). A maximum plasma proguanil concentration of 130.3 +/- 16.0 ng/ml (mean +/- SD) was reached at 3.8 +/- 1.3 h while a maximum cycloguanil concentration of 52.0 +/- 15.2 ng/ml was obtained at 5.3 +/- 1.0 h after dosing. The elimination half-lives of proguanil and cycloguanil were 14.5 +/- 3.0 h and 11.7 +/- 3.1 h, respectively. The plasma clearance of proguanil was 1.43 +/- 0.33 l/h/kg and the apparent volume of distribution was 30.7 +/- 12.3 l/kg. Renal clearance of proguanil (0.33 +/- 0.19 l/h/kg) was about 23% of the plasma clearance and 35.6 +/- 9.6% of the oral dose was recovered as proguanil and cycloguanil.

Antimalarial activity of Mannich bases derived from 4-(7'-bromo-1',5'-naphthyridin-4'-ylamino)phenol and 4-(7'-trifluoromethylquinolin-4'-ylamino)phenol against Plasmodium falciparum in vitro.

Mono- and di-Mannich bases derived from 4-(7'-bromo-1',5'-naphthyridin-4'-ylamino)phenol and 4-(7'-trifluoromethylquinolin-4'-ylamino)phenol were assayed for antimalarial activity (using an in vitro radioisotopic technique) against two isolates of Plasmodium falciparum. Many from these two series of compounds had an IC50 value (concentration of compound causing 50% inhibition of 3H-hypoxanthine incorporation) comparable to or better than those of mefloquine and amodiaquine, for both a chloroquine-sensitive isolate (FCQ-27) and the chloroquine-resistant isolate (K1). At least one compound, 2,6-bis (piperidin-1''-ylmethyl)-4-(7'-trifluoromethylquinolin -4'-ylamino)phenol (TN112), showed significant superior activity to the three antimalarials chloroquine, mefloquine and amodiaquine against both isolates. (Statistically superior activity compared to these three antimalarials was found for TN112, except that against the K1 isolate its activity was just outside the range of significance relative to mefloquine.) Some of the 7-bromo-1,5-naphthyridine Mannich bases were appreciably less toxic in mice than amodiaquine.

Synergistic antimalarial activity of dapsone/dihydrofolate reductase inhibitors and the interaction of antifol, antipyrimidine and antipurine combinations against Plasmodium falciparum in vitro.

Using low folate, low p-aminobenzoic acid medium, 2 isolates of Plasmodium falciparum were tested in vitro against a wide range of antimetabolite compounds with known or potential antimalarial activity. ID50 values (the concentration of compound causing 50% inhibition of [3H]hypoxanthine incorporation) were determined for each compound against both isolates. The compounds tested may affect folate, pyrimidine or purine metabolism in malaria parasites and various combinations of compounds were examined for further synergistic antimalarial effects. The combination of any of the dihydrofolate reductase inhibitors cycloguanil, pyrimethamine or WR 99210 with the sulphone drug dapsone demonstrated strongly synergistic antimalarial activity. Combinations of dihydrofolate reductase inhibitors with the antipyrimidine compounds pyrazofurin or menoctone, or with the antipurine compounds tubercidin, bredinin or hadacidin, or with primaquine, failed to demonstrate synergistic activity. Most combinations of an antipurine with an antipyrimidine compound also failed to show any synergistic effect. However, weak synergism was consistently seen in the tubercidin/pyrazofurin and tubercidin/menoctone combinations. Over the 48 h intraerythrocytic cycle using tightly synchronized parasites, tubercidin demonstrated both a cytotoxic and a cytostatic effect.

In vitro inhibition of Plasmodium falciparum by pyrazofurin, an inhibitor of pyrimidine biosynthesis de novo.

The effect of pyrazofurin, an inhibitor of UMP synthesis, on Plasmodium falciparum growth in vitro has been studied. ID50 values (concentration of compound causing 50% inhibition of [3H]hypoxanthine incorporation) for the FCQ-27, FCI-1 and K-1 (chloroquine-resistant) isolates were 10 +/- 8.7, 6.4 +/- 5.3 and 6.3 +/- 0.5 microM, respectively. Comparative ID50 values for chloroquine were 13.5 +/- 4.2, 22.8 +/- 7.6 and 343 +/- 114 microM, respectively. Over the 48-h intraerythrocytic cycle of tightly synchronized parasites, pyrazofurin both reduced the parasitemia and retarded the maturation of trophozoites and schizonts. Addition of uracil or uridine to the in vitro culture did not decrease the anti-parasitic activity of pyrazofurin. Chloroquine reduced the parasitemia, but did not retard development of the remaining viable parasites. Pyrazofurin (20 microM) caused a 50% inhibition of parasite orotate phosphoribosyltransferase (E.C. 2.4.2.10) and, in the presence of adenosine kinase and ATP, a 73% inhibition of orotidine-5'-phosphate decarboxylase (E.C. 4.1.1.23).