Imidazoquines as antimalarial and antipneumocystis agents

Abstract

Peptidomimetic imidazolidin-4-one derivatives of primaquine (imidazoquines) recently displayed in vitro activity against blood schizonts of a chloroquine-resistant strain of Plasmodium falciparum. Preliminary studies with a subset of such imidazoquines showed them to both block transmission of P. berghei malaria from mouse to mosquito and be highly stable toward hydrolysis at physiological conditions. This prompted us to have deeper insight into the activity of imidazoquines against both Plasmodia and Pneumocystis carinii, on which primaquine is also active. Full assessment of the in vivo transmission-blocking activity of imidazoquines, in vitro tissue-schizontocidal activity on P. berghei-infected hepatocytes, and in vitro anti-P. carinii activity is now reported. All compounds were active in these biological assays, with generally lower activity than the parent drug. However, imidazoquines' stability against both oxidative deamination and proteolytic degradation suggest that they will probably have higher oral bioavailability and lower hematotoxicity than primaquine, which might translate into higher therapeutic indexes.

Figure 1. Flow cytometry-based Plasmodium liver stage infection assays

A, Representative dot plots (left) and histograms (right) of a control sample of Huh7 cells infected with GFP-expressing P.berghei parasites, analysed 48 hours after sporozoite addition; the GFP-positive cells, in a total of 50000 acquired events, corresponds to fraction of infected cells (left), while the geometric mean of the distribution of fluorescence intensities in the GFP-positive population corresponds to the intracellular development of the parasites (right). B, Dose-dependencies of parasite development in cells infected in the presence of increasing concentrations of the various compounds tested, relative to that of solvent-treated samples in the same experiment. C, Example of the calculation of the IC₅₀ for inhibition of parasite development; left- representative histograms of the distribution of fluorescence intensities in the GFP-positive population of cells infected in the presence of different concentrations of compound and of the corresponding solvent control; centre- geometric means of fluorescence intensities relative to that of the solvent-treated control; right- sigmoidal curve of fluorescence intensities as a function of the logarithm of compound concentration; the example pertains the parent compound primaquine (green), with the solvent-treated control shown in red and the value of the IC₅₀ shown as an orange sphere; only 4 out of 6 datapoints are shown to facilitate vieweing of the histogram lines.

Scheme 1

Chemical structures for primaquine and relevant derivatives (see text)

Scheme 2

Synthetic route to imidazoquines 5 and their acetyl analogues 6 and 7. Reagents and conditions: (i) 1 equiv BocAAOH, 1.1 equiv DIC, 1.1 equiv HOBt, 1 equiv triethylamine (TEA), dichloromethane (DCM), 0 °C→rt; (ii) TFA, rt, 30% aq Na₂CO₃, extraction with CHCl₃; (iii) excess propanone, 1 eq. TEA, molecular sieves, refluxing MeOH; (iv) 5 equiv BocAAOH, 5 equiv DIC/HOBt, 3 equiv TEA, DMF, -10 °C→rt, inert atmosphere; (v) TFA/DCM 30%, rt, followed by Na₂CO₃ aq 30% and extraction with DCM; (vi) 5 equiv Ac₂O, 5 equiv DIC/HOBt, 3 equiv TEA, DMF, - 10 °C→rt, inert atmosphere; (vii) refluxing neat Ac₂O (20 equiv).