132 Parasitology Today, vol. 15, no. 4, 1999 that parasitaemia failed to clear in seven of these cases, where parasites before treatment were either wild type or S108N (mutated at codon 108 alone), although in four out of seven cases, the initial parasitaemia contained parasites mutated at dhps, which ‘would thus have been a better indication of outcome’. However, in six out of seven of these cases, parasitaemia (or parasite DNA) did clear following PSD treatment, but reappeared (as a result of either recrudescence or reinfection) at varying times up to Day 28. It appears that Sims et al. have regarded ‘early reappearance’of parasitaemia (or parasite DNA) in patients who initially clear parasites as synonymous with ‘failure to clear’. Our model predicts that PSD will clear peripheral parasitaemia in infections that are either wild type or S108N, which was the case in the study quoted by Sims et al. Conversely, our model ascribes borderline sensitivity to PSD treatment in parasites that are triply mutant in dhfr. We note that the UMIST group also considers this a possibility15. Finally, we feel that there is still confusion surrounding the significant difference in SDX IC50 between that measured in the low-folate, in vitro system, and that of the in vivo state. For example, in paragraph four of their comments, Sims et al. quote the SDX IC50 in low folate for ‘the most highly SDX-resistant line we have examined’, of 13 μM lJ1, and that this is below the maximum achievable serum level, following PYR–SDX treatment, of 25 μM lJ1. As a result,‘we would thus expect this component of the clinical combination to show some activity, even against such highly resistant parasites…’. The point is that the SDX IC50 for these same parasites, in vivo, will be in the millimolar, rather than micromolar range: SDX at 25 μM lJ1, alone, would be virtually inactive, although still capable of synergic action in the presence of pyrimethamine. In this latter statement, and in others (paragraphs 5 and 7 of Ref. 1), the UMIST group give the impression that we do not consider the SDX component of importance to treatment efficacy. The converse is true; in the opening paragraph of our paper, we clearly state our conviction that only when the two drugs act together on the parasite is therapeutic antiparasitic activity achieved. What we have questioned is the location of the active site for the sulfonamide component in synergic action; the evidence from our isobole studies is that this is not parasite DHPS. If true, this point is extremely important in the development of future antifolate antimalarials, because the focus of activity would concern a single enzyme site: DHFR.

In conclusion, we are very glad that Sims and his group have entered into this debate with such enthusiasm; such debate represents the only way forward in this important but under-researched area. It seems to us quite likely that our differences in concept are more apparent than real, and could probably be resolved during an intensive workshop.