Inverse Correlation between Lethality and Thermodynamic Stability of Contact Insecticide Polymorphs

Contact insecticides often involve the interaction of whole organisms and toxicant crystal surfaces. The crystalline phase of these compounds has not, however, been considered for the optimization of insecticide performance. Lindane (1R,2r,3S,4R,5r,6S-hexachlorocyclohexane) has been one of the most widely used insecticides, but other (inactive) stereoisomers accompanying its manufacture have led to massive chemical waste remediation problems. Crystalline polymorphs are also isomers in the broadest sense, yet only one crystal structure of lindane has been reported. Herein, we report the discovery and characterization of two new polymorphs, Forms II and III. The efficacy of Forms I, II, and III against Drosophila melanogaster revealed an inverse correlation between lethality and thermodynamic stability; the least stable kills fastest. This understanding provides a crystal engineering opportunity wherein formulations containing the most active contact insecticide polymorph can achieve infectious disease prophylaxis while reducing environmental exposure and associated chemical waste.