Rational modification of a candidate cancer drug for use against Chagas disease

Abstract

Chagas disease is one of the major neglected diseases of the world. Existing drug therapies are limited, ineffective, and highly toxic. We describe a novel strategy of drug discovery of adapting an existing clinical compound with excellent pharmaceutical properties to target a pathogenic organism. The protein farnesyltransferase (PFT) inhibitor tipifarnib, now in phase III anticancer clinical trials, was previously found to kill Trypanosoma cruzi by blocking sterol 14 alpha-demethylase (14DM). We rationally developed tipifarnib analogues that display reduced affinity for human PFT to reduce toxicity while increasing affinity for parasite 14DM. The lead compound has picomolar activity against cultured T. cruzi and is efficacious in a mouse model of acute Chagas disease.

Figure I

Tipifarnib analog series ring numbering, tipifarnib and Compound 2g.

Figure II

(a) Mammalian PFT depicted with bound compound 2c. Compound 2c is depicted instead of the lead 2g in order to show water-mediated hydrogen bonding between the ammonium group of tipifarnib or compound 2c and the β-phosphate oxygen, which are shown in dotted lines. Compound 2c, bridging water, and the diphosphate of farnesyl diphosphate are rendered as sticks. The prenyl chain is shown as yellow spheres. All van der Waal surfaces are displayed as doubled-radius surfaces to show ligand contacts. The superimposed arrow indicates the clash between the 2-methyl group of compound 2c and the PFT surface. PFT α-subunit and residues R291 to K294 have been removed from the picture for clarity. (b) T. cruzi 14DM with bound compound 2c and heme (both displayed as sticks) showing the accommodation of the inhibitor's 2-methyl group. An 11 Å sphere around the inhibitor is depicted and residues P93-N102, F103-T109 and F386-V406 removed from figure for clarity. All van der Waal surfaces are displayed as doubled-radius surfaces to show ligand contacts.

Figure II

(a) Mammalian PFT depicted with bound compound 2c. Compound 2c is depicted instead of the lead 2g in order to show water-mediated hydrogen bonding between the ammonium group of tipifarnib or compound 2c and the β-phosphate oxygen, which are shown in dotted lines. Compound 2c, bridging water, and the diphosphate of farnesyl diphosphate are rendered as sticks. The prenyl chain is shown as yellow spheres. All van der Waal surfaces are displayed as doubled-radius surfaces to show ligand contacts. The superimposed arrow indicates the clash between the 2-methyl group of compound 2c and the PFT surface. PFT α-subunit and residues R291 to K294 have been removed from the picture for clarity. (b) T. cruzi 14DM with bound compound 2c and heme (both displayed as sticks) showing the accommodation of the inhibitor's 2-methyl group. An 11 Å sphere around the inhibitor is depicted and residues P93-N102, F103-T109 and F386-V406 removed from figure for clarity. All van der Waal surfaces are displayed as doubled-radius surfaces to show ligand contacts.

Figure III. Pharmacokinetics and efficacy of tipifarnib and 2g in mice

(a) Plasma levels were monitored following a single oral dose of 50 mg/kg in uninfected mice. Plotted values are an average of three mice for both tipifarnib and compound 2g. (b) Efficacy was monitored by measuring parasitemia in T. cruzi infected mice receiving treatment with tipifarnib (50 mg/kg twice daily), compound 2g (50 mg/kg twice daily), vehicle (twice per day), or benznidazole (100 mg/kg once daily). Treatments were administered by oral gavage for 20 consecutive days beginning on day 8 post-infection with T. cruzi trypomastigotes. Vehicle treated mice (negative control) were all dead by day 16 post-infection.

Scheme Ia. Synthesis of Tipifarnib Analogs from Phenylacetonitrile

a) p-bromonitrobenzene, NaOH, MeOH, 10−50% b) TiCl₃, H₂O/THF, rt 62% c) Ac₂O, toluene, reflux d) t-BuOK, DME 20 °C, 66% (2 steps) e) BF₄OMe₃, DCM, 63% f) i.) n-BuLi, THF, −78 °C ii. (11a-b) 60% g) 6N HCl, THF, reflux 6 hr 60% h) CH₃I, NaOH, BTEAC, THF, rt 66% i) SOCl₂, neat, 12 hrs j) NH₃, THF, rt 52% (2 steps)

Scheme Ib. Synthesis of (Imidazol-5-yl)phenylmethanone Intermedate (11)

a) SOCl₂, neat, rt b) CH₃ONHCH₃, pyridine, DCM, 90% (2 steps) c) N-methylimidazole, i.) n-BuLi, THF, −78 °C ii.) Et₃SiCl, −78 °C iii.) n-BuLi, THF, −78 °C, 77%

Scheme II. Synthesis of 3-Chloro-2-methylphenylacetonitrile Intermediate

a) LAH, THF, quant. b) PBr₃, DCM, 90% c) NaCN, DMSO, 85% d) p-BrPhNO₂, NaOH, MeOH, 10% e) TiCl₃, H₂O/THF, rt, 50% (4% overall)

Scheme III. Synthesis of Tipifarnib Analogs from 5-Bromoisatoic anhydride

a) CH₃ONHCH₃ HCl, pyridine, DCM, 83% b) RPhBr (2eq.), n-butyllithium (2eq.), THF, 85% c) Ac₂O, toluene, reflux d) t-BuOK, DME 20 °C, 66% (2 steps) e) BF₄OMe₃, DCM, 63% f) i.) n-BuLi, THF, −78 °C ii. (11a-c), 60% g) 6N HCl, THF, reflux, 60% h) CH₃I, NaOH, BTEAC, THF, rt, 66% i) SOCl₂, neat, 12 hrs j) NH₃, THF, rt, 52% (2steps) k) tosic acid (1eq + cat.), MeOH, reflux, 75%.

Chart I

Structures of Tipifarnib 1 and Tipifarnib analogs 2a-g.