Substituent effects on desferrithiocin and desferrithiocin analogue iron-clearing and toxicity profiles

Abstract

Desferrithiocin (DFT, 1) is a very efficient iron chelator when given orally. However, it is severely nephrotoxic. Structure-activity studies with 1 demonstrated that removal of the aromatic nitrogen to provide desazadesferrithiocin (DADFT, 2) and introduction of either a hydroxyl group or a polyether fragment onto the aromatic ring resulted in orally active iron chelators that were much less toxic than 1. The purpose of the current study was to determine if a comparable reduction in renal toxicity could be achieved by performing the same structural manipulations on 1 itself. Accordingly, three DFT analogues were synthesized. The iron-clearing efficiency and ferrokinetics were evaluated in rats and primates; toxicity assessments were carried out in rodents. The resulting DFT ligands demonstrated a reduction in toxicity that was equivalent to that of the DADFT analogues and presented with excellent iron-clearing properties.

Figure 1

Job’s plots of the Fe(III) complex of ligand 6, 8, and 10. Solutions containing different ligand/Fe(III) ratios were prepared such that [ligand] + [Fe(III)] = 1.0 mM in Tris-HCl buffer at pH 7.4. The theoretical mole fraction maximum for a 2:1 ligand:Fe complex is 0.667 (arrow). The observed maxima for 6, 8, and 10 are 0.669, 0.676, and 0.677, respectively. Optical density (y-axis) was determined at 498, 484, and 485 nm for 6, 8, and 10, respectively.

Figure 2

Biliary ferrokinetics of DFT analogues (6, 8, 10) and DADFT analogues (3, 7, 9) in bile duct-cannulated rats. The compounds were given po at 300 µmol/kg. The iron excretion (y-axis) is reported as µg of iron per kg body weight.

Figure 3

Iron-clearing efficiency, expressed as a percentage (y-axis), versus log P_app for DFT analogues 6, 8, and 10 in bile duct-cannulated rats given po at a dose of 300 µmol/kg.

Figure 4

Urinary Kim-1 excretion (y-axis) is expressed as Kim-1 (ng/kg/24 h) of rats treated with DFT (1), DFT analogues 6, 8, and 10 or DADFT analogues 3 and 7. The rodents were given the drugs po twice daily (b.i.d.) at a dose of 237 µmol/kg/dose (474 µmol/kg/d) for up to 7 d. Note that none of the rats survived the planned 7-d exposure to 1. N = 5 for 1, 6–8, and 10; N = 3 for ligand 3.

Scheme 1

Synthesis of 6 and 8a ^aReagents and conditions: (a) 4-methoxybenzyl alcohol, 60% NaH (2.5 equiv each), DMF, 95–100 °C, 18 h, 73%; (b) TFA, pentamethylbenzene, 22 h, quantitative; (c) CH₃OH, 0.1 M pH 6 buffer, NaHCO₃, 73–76 °C, 45 h; (d) EtI, DIEA (1.5 equiv each), DMF, 47 h, 70%; (e) K₂CO₃ (1.6 equiv), acetone, reflux, 1 d, 65%; (f) 50% NaOH (aq), CH₃OH, then HCl, 96% (6), 97% (8).

Scheme 2

Synthesis of 10a ^aReagents and conditions: (a) K₂CO₃ (2 equiv), CH₃CN, 68%; (b) m-CPBA, CH₂Cl₂; (c) Ac₂O, reflux; (d) NaOH (aq), EtOH, reflux, 4h, 87%; (e) SO₃˙pyridine, NEt₃, DMSO, CHCl₃, 16 h, 83%; (f) H₂NOH˙HCl, NaOAc, CH₃OH, reflux, 2 h, 90%; (g) Ac₂O, reflux, 94%; (h) H₂, 10% Pd-C, CH₃OH, 85%; (i) CH₃OH, 0.1 M pH 6 buffer, NaHCO₃, 75 °C, 48 h, 95%.

Chart 1

Extensive structural alterations of the tridentate chelator (S)-4,5-dihydro-2-(3-hydroxy-2-pyridinyl)-4-methyl-4-thiazolecarboxylic acid (1) were carried out, including simple hydroxylation of the aromatic ring of (S)-4,5-dihydro-2-(2-hydroxyphenyl)-4-methyl-4-thiazolecarboxylic acid (2) to yield (S)-2-(2,4-dihydroxyphenyl)-4,5-dihydro-4-methyl-4-thiazolecarboxylic acid (3), or the introduction of polyether fragments to 2, e.g., (S)-4,5-dihydro-2-[2-hydroxy-4-(3,6,9-trioxadecyloxy)]-4-methyl-4-thiazolecarboxylic acid (4), and (S)-4,5-dihydro-2-[2-hydroxy-3-(3,6,9-trioxadecyloxy)]-4-methyl-4-thiazolecarboxylic acid (5).