Potent Antiplasmodial Derivatives of Dextromethorphan Reveal the Ent-Morphinan Pharmacophore of Tazopsine-Type Alkaloids

Abstract

The alkaloid tazopsine 1 was introduced in the late 2000s as a novel antiplasmodial hit compound active against Plasmodium falciparum hepatic stages, with the potential to develop prophylactic drugs based on this novel chemical scaffold. However, the structural determinants of tazopsine 1 bioactivity, together with the exact definition of the pharmacophore, remained elusive, impeding further development. We found that the antitussive drug dextromethorphan (DXM) 3, although lacking the complex pattern of stereospecific functionalization of the natural hit, was harboring significant antiplasmodial activity in vitro despite suboptimal prophylactic activity in a murine model of malaria, precluding its direct repurposing against the disease. The targeted N-alkylation of nor-DXM 15 produced a small library of analogues with greatly improved activity over DXM 3 against P. falciparum asexual stages. Amongst these, N-2'-pyrrolylmethyl-nor-DXM 16i showed a 2- to 36-fold superior inhibitory potency compared to tazopsine 1 and DXM 3 against P. falciparum liver and blood stages, with respectively 760 ± 130 nM and 2.1 ± 0.4 μM IC₅₀ values, as well as liver/blood phase selectivity of 2.8. Furthermore, cpd. 16i showed a 5- to 8-fold increase in activity relative to DXM 3 against P. falciparum stages I-II and V gametocytes, with 18.5 μM and 13.2 μM IC₅₀ values, respectively. Cpd. 16i can thus be considered a promising novel hit compound against malaria in the ent-morphinan series with putative pan cycle activity, paving the way for further therapeutic development (e.g., investigation of its prophylactic activity in vivo).

Keywords: N-alkylation; Plasmodium berghei; Plasmodium falciparum; blood stages; dextromethorphan; hepatic stages; hit compound; malaria; prophylaxis; tazopsine.

Figure 1

Antiplasmodial ent-morphinans: natural tazopsine 1, semisynthetic hit 2, and the prospected DXM 3.

Figure 2

Semisynthetic access to tazopsine derivatives 6–8 and structure of natural ent-morphinan alkaloids for SAR generation (morphine numbering). (a) CH₂N₂, MeOH, 0 ${}^{\circ }$C, 12 h (39%); (b) aldehyde (1.1 eq), MeOH, r. t., 10 min then NaBH₃CN (1 eq), r. t. (45–82%); (c) Ac₂O (1 eq), MeOH, r. t., 1 h (45%). The box indicates the natural alkaloids.

Figure 3

In vivo prophylactic activity of DXM 3 +/− QND compared to PQ in a mouse model of P. yoelii-Luc infection. Each group of mice consisted of five individuals. Results are shown as mean +/− SEM: Vehicle vs. DXM40 + QND20, p = 0.5171 (ns, not significative); Vehicle vs. PQ, p = 0.0124 (*); DXM40 vs. PQ and QND20 vs. PQ, p = 0.003 (**). ANOVA one-way test for multiple comparisons. Numeric values indicate doses in mg/kg.

Figure 4

Semisynthetic access to DXM derivatives 11–18 for SAR generation (morphine numbering). (a) NIS (1.15 eq), p-TsOH (1.8 eq) (91% 11, 87% 13); (b) HBr 48%, reflux overnight (91%); (c) 2′,2′,2′-trichloroethylchloroformate (1.1 eq), toluene, reflux, 2 h (92%); (d) Zn powder (3 eq), AcOH, r. t., 1 h (47%); (e) aldehyde (1.1 eq), DMF, r. t., 10 min then STABH (2 eq), r. t. (33–97%); (f) cyclopropanecarbonyl chloride (1.1 eq), CH₂Cl₂, Et₃N, 0 ${}^{\circ }$C to r. t, 1.5 h (75%); (g) MeI, rt, 2 h (82%).

Figure 5

Normalized dose–response heatmap in PHH-Pb-GFP regarding EEF number (A) and EEF size (B) at 0.1, 1, 10, or 20 $\mathsf{\mu }$M of the indicated compounds. These results were obtained from four technical replicates.

Figure 6

Normalized EEF number (A) and EEF size (B) in PHH-Pb-GFP at the arbitrary cut-off value of 7 μM for the indicated compounds. Each point (•) represents the average EEF number of four technical replicates. The error bars represent the interval of variability between EEF numbers. These results were obtained from four technical replicates.

Figure 7

Inhibitory plots and IC₅₀ values of DXM 3 (purple) and cpd. 16i (cyan) against P. falciparum early (stages I–II) (A) and late (stage V) (B) gametocytes in vitro. IC₅₀ values are the mean of three technical replicates and three biological replicates. Error bars represent standard deviation.