A comparison of the in vitro activities of amodiaquine and desethylamodiaquine against isolates of Plasmodium falciparum.

The antimalarial activities of amodiaquine, the desethyl metabolite of amodiaquine, chloroquine, and mefloquine were evaluated against 35 field isolates of Plasmodium falciparum collected from eastern Thailand, October-December 1985, to define patterns of cross-resistance among these compounds. The assay system was based on the in vitro inhibition of schizont maturation. The parasites were generally sensitive to mefloquine (mean 50%-inhibitory concentrations = 9.98 nM) and highly resistant to chloroquine (IC50 = 313 nM). The mean in vitro activity of desethylamodiaquine (67.5 nM) was approximately 3.5 times lower than that of amodiaquine (18.2 nM). There was a significant rank-order correlation between the IC50S of desethylamodiaquine and chloroquine, but not between amodiaquine and chloroquine, which suggests that the apparent cross-resistance between chloroquine and amodiaquine observed in clinical studies may be more closely related to the cross-resistance between chloroquine and the metabolite rather than between chloroquine and the parent compound. Isolates with IC50 values of amodiaquine greater than 20 nM demonstrated a high degree of correlation with values of desethylamodiaquine; however, it was not possible to accurately predict the sensitivity to desethylamodiaquine of isolates which had IC50 values of amodiaquine of less than 20 nM.

A comparative field study of radiolabeled and enzyme-conjugated synthetic DNA probes for the diagnosis of falciparum malaria.

Synthetic, 21-base, DNA probes to the genome of Plasmodium falciparum were either 32P-labeled or enzyme-conjugated for comparative field studies. The sensitivity of both probes was compared with microscopy in the examination of blood samples from 97 Thai villagers, 47 Thai Rangers, and 19 malaria-free Bangkok residents. The probes were also used to monitor the therapeutic response of 18 of the Rangers during 7 days of treatment. The probes proved highly specific. Both probes had lower limits of detection of about 100 parasites per microliter blood. Thus, the low parasite densities in partially immune villagers from an endemic area were often missed, while higher parasite densities in the nonimmune Rangers were usually detected. As monitors of response to treatment, the probes paralleled microscopy in identifying reversion from positive to negative parasitemia. The enzymelabeled DNA probe as shown to perform similarly to the radiolabeled probe in populations with different malarial immune status and during curative treatment.

Evaluation of an in vitro assay system for drug susceptibility of field isolates of Plasmodium falciparum from southern Thailand.

An in vitro assay system has been developed to evaluate the susceptibility of field isolates of Plasmodium falciparum to standard and new antimalarials. The assay used drugs which were serially diluted in the field and determined effective drug concentrations by quantitating schizont maturation after a variable incubation period. Based on the ID50 values, a series of isolates from Yala in southern Thailand were shown to be resistant to chloroquine (187 nM) but only moderately resistant to amodiaquine (23.7 nM), a structurally related 4-aminoquinoline. Five aminocarbinols were evaluated. The parasites were resistant to quinine (219 nM), but comparatively much more susceptible to mefloquine (9.04 nM), halofantrine (1.23 nM), and enpiroline (6.23 nM). The isolates were also relatively sensitive to WR 194,965 (9.04 nM). Two dihydrofolate reductase inhibitors (WR 99,210 and pyrimethamine) were tested. The isolates were comparatively sensitive to a dihydrotriazine, WR 99,210 (2.85 nM). The in vitro values for pyrimethamine (1,870 nM) were higher than the values for the other drugs tested, but were less than values from other regions of Thailand. As compared to a survey conducted in this region four years previously, values for chloroquine, pyrimethamine, amodiaquine, and mefloquine have remained relatively unchanged. However, there was a greater than 20-fold rise in the susceptibility values for quinine, suggesting the introduction of quinine-resistant isolates from eastern Thailand into southern Thailand during this period.

In vitro activity of pyronaridine against field isolates and reference clones of Plasmodium falciparum.

Pyronaridine, a 9-substituted 1-aza-acridine, was assayed for in vitro activity against clinical and field isolates as well as characterized clones of Plasmodium falciparum. The in vitro antimalarial activity of pyronaridine was compared to activities of standard antimalarials against multidrug-resistant isolates of P. falciparum from eastern and northern Thailand using an assay based on the inhibition of schizont maturation. Isolates from eastern Thailand (n = 30) were susceptible to pyronaridine (IC50 8.40 nM), mefloquine (IC50 6.97 nM), and amodiaquine (IC50 12.7 nM) and resistant to chloroquine (IC50 361 nM), quinine (IC50 388 nM), and pyrimethamine (IC50 11,800 nM). The isolates from northern Thailand (n = 7) showed no statistical difference in susceptibility to pyronaridine (IC50 10.1 nM), amodiaquine (IC50 7.29 nM), and mefloquine (IC50 5.48 nM); however, isolates were significantly more susceptible to chloroquine (IC50 167 nM), quinine (IC50 248 nM), and pyrimethamine (IC50 1,980 nM). These data suggest a lack of cross-resistance between pyronaridine and either chloroquine, quinine, or pyrimethamine. Using the same assay system the in vitro activity of pyronaridine was evaluated against isolates from treatment failures of mefloquine or enpiroline from eastern Thailand. The IC50 values for mefloquine against five recrudescent isolates were significantly higher (IC50 16.4 nM) than the field isolates collected from the same region (IC50 6.97 nM); however, there was no significant difference in the pyronaridine susceptibility between the isolates from the field study (IC50 8.89 nM) and the isolates from the treatment failures (IC50 8.40 nM). These observations suggest a lack of cross-resistance to mefloquine following treatment failure with either mefloquine or enpiroline.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro mefloquine resistance of Plasmodium falciparum isolated from the Burmese border region of Thailand.

The in vitro susceptibility of twenty isolates of Plasmodium falciparum from Tha Song Yang, Tak province, Thailand were determined. The isolates were resistant to chloroquine (IC50 = 220 nM; MIC = 762 nM), quinine (IC50 = 252 nM; MIC = 1010 nM), and pyrimethamine (IC50 = 16400 nM; MIC = 43100 nM) but generally sensitive to mefloquine (IC50 = 6.90 nM; MIC = 20.9 nM) and halofantrine (IC50 = 8.73 nM; MIC = 2.71 nM). Two isolates were identified which appeared resistant to mefloquine (IC50 = 23.1 nM; MIC = 56.6 nM). These isolates may represent an extension of a population of parasites from eastern Thailand.