X-ray Crystallographic Structure of the Potent Antiplasmodial Compound 2,7-Dibromocryptolepine Acetic Acid Solvate

Abstract

The structure of 2,7-dibromocryptolepine acetic acid solvate, C₁₆H₁₁N₂Br₂ [1.5(C₂H₄O₂)][C₂H₃O ⁻₂ ][0.5H₂O], M_r = 460.17, has been determined from X-ray diffraction data. The crystals are monoclinic, space group P2₁/c with Z = 4 molecules per unit cell and a = 7.3243(3), b = 18.7804(6), c = 15.8306(7) Å, β = 94.279(1)°, V_c = 2171.5(2) Å, crystal density D_c = 1.667 g/cm³. The structure was determined using direct methods and refined by full-matrix least-squares to a conventional R-index of 0.0496 for 4,908 reflections and 258 parameters. The cryptolepine nucleus of the 2,7-dibromocryptolepine molecule is highly planar and the two Br atoms are in this plane within 0.06 and 0.01 Å, respectively. The crystal structure is maintained via hydrogen bonding between N(10) in the cryptolepine nucleus and the oxygen of one of the three solvated acetic acid molecules. The acetic acid molecules also form hydrogen bonded chains. Acetic acid B is deprotonated and its two C–O bond lengths are equivalent, unlike those in A and C. Acetic acid C lies very close to a crystallographic centre of symmetry. To avoid overlap the two repeats cannot exist together and are subject to 50% statistical disorder. O(1C) of this methanol is furthest from the two-fold axis and its occupancy refines to a value of 1.0 and is assumed to exist alternately as a water oxygen hydrogen bonding to methanol O(1C) across the two-fold axis at a distance of 2.775 Å.

Index Abstract

The antiplasmodial activity of the analogue 2,7-dibromocryptolepine is nine times greater than that of cryptolepine itself against chloroquine-resistant Plasmodium falciparum (multi-drug resistant strain K1) with IC₅₀ values = 0.44 and 0.049 μM, respectively. This analogue also showed promising in vivo activity against P. berghei in mice. Parasitaemia was suppressed by 91.4% compared to untreated infected control animals at a dose of 25 mg/kg given daily by i.p. injection with no apparent toxicity to the mice, in contrast to cryptolepine which was toxic to mice when given i.p. at 20 mg/kg. Further experiments showed a dose-dependent relationship with ED₅₀ and ED₉₀ values of 6.92 and 21.46 mg/kg/day, respectively. Although 2,7-dibromocryptolepine was not toxic to the mice its cytotoxic activity is similar to that of cryptolepine, but unlike cryptolepine it does not appear to intercalate into DNA as assessed using thermodenaturation techniques (ΔTm values = 3 and 9 °C, respectively). Like cryptolepine, 2,7-dibromocryptolepine inhibits the formation of haemozoin, but its increased antiplasmodial potency does not appear to be due to more potent inhibition of haemozoin formation, nor to increased accumulation into the acidic parasite food vacuole suggesting that 2,7-dibromocryptolepine has additional modes of action compared to cryptolepine. The X-ray structure of this compound will help to determine whether or not 2,7-dibromocryptolepine is able to intercalate into DNA and facilitate the design of new cryptolepine analogues with DNA binding properties appropriate for antimalarial (with no DNA intercalation) or anticancer (sequence-specific binding) applications.