The thiol-based reduction of Bi(V) and Sb(V) anti-leishmanial complexes

Low molecular weight thiols including trypanothione and glutathione play an important function in the cellular growth, maintenance and reduction of oxidative stress in Leishmania species. In particular, parasite specific trypanothione has been established as a prime target for new anti-leishmania drugs. Previous studies into the interaction of the front-line Sb(V) based anti-leishmanial drug meglumine antimoniate with glutathione, have demonstrated that a reduction pathway may be responsible for its effective and selective nature. The new suite of organometallic complexes, of general formula [MAr₃(O₂CR)₂] (M = Sb or Bi) have been shown to have potential as new selective drug candidates. However, their behaviour towards the critical thiols glutathione and trypanothione is still largely unknown. Using NMR spectroscopy and mass spectrometry we have examined the interaction of the analogous Sb(V) and Bi(V) organometallic complexes, [SbPh₃(O₂CCH₂(C₆H₄CH₃))₂] S1 and [BiPh₃(O₂CCH₂(C₆H₄CH₃))₂] B1, with the trifluoroacetate (TFA) salt of trypanothione and L-glutathione. In the presence of trypanothione or glutathione at the clinically relevant pH of 4-5 for Leishmania amastigotes, both complexes undergo facile and rapid reduction, with no discernible difference. However, at a higher pH (6-7), the complexes behave quite differently towards glutathione. The Bi(V) complex is again reduced rapidly but the Sb(V) complex undergoes slow reduction over 8 h (t_1/2 = 54 min.) These results give the first insights into why the highly oxidising Bi(V) complexes display low selectivity in their cytotoxicity towards leishmanial and mammalian cells, while the Sb(V) complexes show good selectivity.