Synthesis and Biological Evaluation of Substrate-Based Inhibitors of 6-Phosphogluconate Dehydrogenase as Potential Drugs Against African Trypanosomiasis Christophe Dardonville, a Eliana Rinaldi, b Stefania Hanau, b Michael P. Barrett, c Reto Brun d and Ian H. Gilbert a, * a Welsh School of Pharmacy, Redwood building, Cardiff University, King Edward VII Avenue, Cardiff CF10 3XF, UK b Dipartimento di Biochimica e Biologia Molecolare, Universita’ di Ferrara, Via Luigi Borsari 46, 44100 Ferrara, Italy c IBLS, Division of Infection & Immunity, University of Glasgow, Glasgow G12 8QQ, UK d Swiss Tropical Institute, Socinstrasse 57, CH-4002 Basel, Switzerland Received 10 January 2003; accepted 21 March 2003 Abstract—The synthesis and biological evaluation of three series of 6-phosphogluconate (6PG) analogues is described. (2R)-2- Methyl-4,5-dideoxy, (2R)-2-methyl-4-deoxy and 2,4-dideoxy analogues of 6PG were tested as inhibitors of 6-phosphogluconate dehydrogenase (6PGDH) from sheep liver and also Trypanosoma brucei where the enzyme is a validated drug target. Among the three series of analogues, seven compounds were found to competitively inhibit 6PGDH from T. brucei and sheep liver enzymes at micromolar concentrations. Six inhibitors belong to the (2R)-2-methyl-4-deoxy series (6, 8, 10, 12, 21, 24) and one is a (2R)-2- methyl-4,5-dideoxy analogue (29b). The 2,4-dideoxy analogues of 6PG did not inhibit both enzymes. The trypanocidal effect of the compounds was also evaluated in vitro against T. brucei rhodesiense as well as other related trypanosomatid parasites (i.e., Trypano- soma cruzi and Leishmania donovani). # 2003 Elsevier Science Ltd. All rights reserved. Introduction Human African trypanosomiasis is a major health problem in sub-Saharan Africa, caused by, Trypano- soma brucei gambiense in West and Central Africa and T.b. rhodesiense in East Africa. 1,2 The drugs currently used to treat sleeping sickness are far from satisfactory because of major side effects, 3 the parenteral mode of administration, increasing resistance 4 and the unafford- able price for African countries. Moreover, some of these drugs (suramin, pentamidine) are unable to cross the blood–brain barrier in sufficient quantity to treat late-stage cases of HAT. 5 Bloodstream forms of T. brucei produce ATP exclu- sively through glycolysis, making the inhibition of any of the glycolytic enzymes a potential therapeutic approach. 6 We decided to target the enzyme 6-phos- phogluconate dehydrogenase (6PGDH), the third enzyme of the pentose phosphate pathway 7 (PPP) which generates NADPH and ribulose-5-phosphate (Fig. 1). The PPP plays a crucial role in the host–parasite rela- tionship because it maintains a pool of NADPH, which amongst other roles is involved in protecting the para- site against oxidative stress, and it generates carbohy- drate intermediates used in nucleotide and other biosynthetic pathways. The gene encoding the T. brucei 6PGDH has been cloned, 8 and the enzyme purified 9 and crystallized. The enzyme is remarkable for its degree of divergence with regard to other eukaryotic 6PGDHs. This can be 0968-0896/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0968-0896(03)00191-3 Bioorganic & Medicinal Chemistry 11 (2003) 3205–3214 Figure 1. *Corresponding author. Fax.: +44-(0)29-2087-4149; e-mail: gilber- tih@cf.ac.uk