Re-engineering aryl methylcarbamates to confer high selectivity for inhibition of Anopheles gambiae versus human acetylcholinesterase

Abstract

To identify potential human-safe insecticides against the malaria mosquito we undertook an investigation of the structure-activity relationship of aryl methylcarbamates inhibitors of acetylcholinesterase (AChE). Compounds bearing a β-branched 2-alkoxy or 2-thioalkyl group were found to possess good selectivity for inhibition of Anopheles gambiae AChE over human AChE; up to 530-fold selectivity was achieved with carbamate 11d. A 3D QSAR model is presented that is reasonably consistent with log inhibition selectivity of 34 carbamates. Toxicity of these compounds to live Anopheles gambiae was demonstrated using both tarsal contact (filter paper) and topical application protocols.

Figure 1

Thiol-reactive AgAChE-selective inhibitor 1, insecticidal aryl methylcarbamates 2–5, and hAChE-selective carbamate rivastigmine 6.

Figure 2

Plot of k_obs vs [11d] at both AgAChE and hAChE. Second-order rate constants for inactivation k_i (mM⁻¹ min⁻¹) derive from the slope of each line.

Figure 3

Predicted vs observed Log AgAChE/hAChE selectivity of inhibitors in Tables 1–3 except 20w (33 points); an R² value of 0.66 is calculated. Note that prediction for each compound was made with a PLS model trained on a dataset excluding that particular compound, i.e. this is a leave-one-out prediction validation. Subsequently predicted vs. actual Log selectivity was determined for 20w (indicated by arrow).

Figure 4

3D QSAR lipophilic field visualizations for the Ag/h inhibition selectivity of the compounds in Tables 1–3. Lipophilic occupancy of blue regions improves Ag/h inhibition selectivity, and lipophilic occupancy of red regions decreases selectivity. 20w is shown in stick; other compounds are shown in CPK-colored wire (carbon is cyan).

Scheme 1

Synthesis of 2-substituted aryl methylcarbamates and unsubstituted control 17 i) Cs₂CO₃, R-Br (or Cl, I, OTs), DMF, 80 °C, 16 h. ii) NaHCO₃, R-Br (or Cl, I, OTs), DMF, 50 °C, 16 h. iii) KOt-Bu, THF; MeNHC(O)Cl. iv) i-PrMgCl, THF, −78 °C; MeO(Me)NC(O)CH₂CH(Et)₂. v) BBr₃, CH₂Cl₂, 0°C. vi) NaBH₄, CH₃OH; Pd/C, H₂, CH₃OH.

Scheme 2

Synthesis of 3-substituted aryl methylcarbamates 20h–w. NHMe i) SOCl₂, 0°C. ii) AlMe₃, CH₂Cl₂, 0 °C. iii) NaH, THF, MsCl. iv) BBr₃, CH₂Cl₂.v) KOt-Bu, THF; CH₃NHC(O)Cl. vi) 2.1 equiv n-BuLi, THF, −78 to 0 °C; 2 equiv R³-Br or R₃-Cl (for 19s, t); aq. HCl. vii) DMAP, EDCI, HN(OMe)Me, CH₂Cl₂. viii) i-PrMgCl, Et₂O, reflux. ix) NH₂NH₂, EtOH; KOH, ethylene glycol, 160 °C. x) NaOMe, BrCH₂CHEt₂, MeOH. xi) 2.1 equiv n-BuLi, THF, −78 to 0 °C; Me(MeO)NC(O)CH₂CHMe₂.