Michael David Ward

Conspectus: Combatting malaria, a disease afflicting more than 250 million people annually according to the World Health Organization (WHO), requires insecticides as part of integrated vector management. Indoor residual spraying (IRS) of crystalline contact insecticides and insecticide-treated bed nets (ITNs) decorated with insecticide crystals are estimated to have reduced malaria mortality in Africa by 60% in the 21st century. However, resistance now threatens malaria control, spurring the development of new insecticides, a process that requires substantial resources, anddiscovery time while posingenvironmental risks. Improving the effectiveness of compounds currently in use may be preferable. This Account provides a brief history of contact insecticides and describes discoveries from our laboratory that suggest paths to faster-acting contact insecticides based on the engineering of solid-state forms, either amorphous or crystalline polymorphs, thereby potentially obviating the need for new chemicals. Contact insecticides are thought to affect insects by absorption through the footpads. Prior to our 2017 report on the structure of a second solid form of DDT, crystal polymorphism was not optimized for contact insecticides. Comparative analysis of knockdown times for flies and mosquitoes against polymorphs of well-known crystalline contact insecticides including DDT and its analogs, as well as lindane, deltamethrin, and imidacloprid, established a link between thermodynamic crystal stability and insect knockdown speed. The relationship between the knockdown by a crystalline contact insecticide and its crystal structure ultimately arises from crystal thermochemistry. Weak intermolecular interactions in complex systems and associated shallow potential energy hypersurfaces readily lead to crystalline polymorphs with different molecular organizations that vary in crystal free energies and associated bioavailabilities. Notably, a new form of deltamethrin was 12 times more active than the commercial form, and the least stable polymorph of imidacloprid was six and nine times more active against susceptible Anopheles and Aedes mosquitoes, respectively, than the commercial, thermodynamically stable form. Some of these metastable polymorphs were found to be stable against transformation to their thermodynamically stable forms for months in an idealized laboratory setting, approaching WHO guidelines for practical use. The observation of differing polymorph effectiveness demonstrates that tarsal absorption of molecules from the crystal surfaces by insects is a key step, and likely a limiting step, in the insecticidal action. Indeed, a persistently amorphous form of deltamethrin dispersed on chalk exhibited dramatically increased efficacy against deltamethrin-resistant Anopheles mosquitoes from Burkina Faso, revealing that an increased rate of insecticide uptake overwhelmed all the resistance mechanisms tested. Collectively, this work argues that manipulation of the solid-state chemistry of contact insecticides is a viable strategy for mitigating insect-borne diseases and one that should be considered along with others in integrated vector management.

Transformation of optically active (G)₂(1,5-NDS)⊃EtOH banded spherulites to guest-free (G)₂(1,5-NDS) straight crystals upon heating.

[This corrects the article DOI: 10.1021/acsmedchemlett.4c00423.].

Hyperoxaluria is caused by increased urinary excretion of oxalate leading to the formation of calcium oxalate (CaOx) stones. The lack of effective management strategies for hyperoxaluria prompted us to investigate molecular mimics as stone inhibitors, a strategy that we previously used successfully to discover small molecule inhibitors of l-cystine crystallization for the prevention of l-cystine stone formation in cystinuria. Herein, we report the discovery of l-lysine dioxalate (LH1513), a novel dioxamate derivative, as a more potent inhibitor of CaOx crystallization than citrate and pyruvate. Such inhibition was corroborated by in situ atomic force microscopy (AFM) measurements of crystal growth rates at the microscopic length scale. A triester prodrug of LH1513 was found to have sufficient oral bioavailability for a preliminary in vivo study demonstrating efficacy in preventing urinary CaOx crystal formation in an Agxt-knockout mouse model for hyperoxaluria.

Encapsulation of liquid guest molecules in hydrogen-bonded frameworks permits analysis of their preferred conformations through single crystal X-ray diffraction.

The activity of crystalline contact insecticides relies on the extraction of surface molecules by insect tarsi upon contact. Most crystals are inherently anisotropic, and surface molecules on symmetry independent faces are expected to have different free energies. The facet-dependent bioavailability and associated efficacy of insect lethality have not been investigated, however. We discriminate the bioactivity of various facets of single crystals of DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane), a well-known contact insecticide. Our findings reveal facet-dependent lethality differences of nearly 75% among four crystallographically unique facets. Furthermore, computations reveal that the respective lethalities of the facets are strongly correlated with the detachment energies of molecules from the crystal surfaces. This facet-dependent lethality suggests a pathway to enhance the efficacy of known contact insecticides through crystal habit control.

Cystinuria, a rare genetic disorder, is characterized by defective L-cystine reabsorption from the renal proximal tubule, resulting in abnormally high concentrations of l-cystine and subsequent L-cystine crystallization in urine and stone formation in the urinary tract. Inhibition of L-cystine crystallization by L-cystine diamides such as LH708 (2) represents a promising new approach to prevent stone formation in patients with cystinuria. While 2 shows promising in vivo efficacy and a good safety profile in a Slc3a1-knockout mouse model of cystinuria, further structural modification of 2 led to the discovery of 8-L-cystinyl bis(1,8-diazaspiro[4.5]decane) (LH1753, 3) incorporating a bioisosteric spiro bicyclic diamine 1,8-diazaspiro[4.5]decane for the N-methylpiperazine terminal groups in 2 as a promising candidate with 3 being about 120x more potent than l-cystine dimethyl ester (CDME, 1) and about 2x more potent than 2 in inhibiting l-cystine crystallization. Furthermore, 3 demonstrated good oral bioavailability and in vivo efficacy in preventing l-cystine stone formation in the Slc3a1-knockout mouse model of cystinuria.

Cystinuria is a rare genetic disorder characterized by defective l-cystine reabsorption from the renal proximal tubule, resulting in abnormally high concentrations of L-cystine and subsequent l-cystine crystallization and stone formation in urine. l-Cystine diamides have shown great promise as inhibitors of l-cystine crystallization. The free α-amino groups in l-cystine diamides have previously been shown to be necessary for l-cystine crystallization inhibitory activity. In this study, three additional series of l-cystine diamide analogs were designed to explore further the structure-activity relationships for l-cystine crystallization inhibition. It has been demonstrated that the middle disulfide bond is required for optimal l-cystine crystallization inhibitory activity, and the only regions that can be modified are the two terminal amides. The presence of another basic amine 2–3 atoms away from the amide nitrogen is also critical for optimal activity. Disulfide exchange was found to be the main metabolic pathway resulting in the formation of two molecules of the active mixed disulfide metabolite from a single l-cystine diamide. l-Cystine diamides have the potential to be developed into a much-needed therapy for cystinuria.

Single crystal X-ray diffraction (SCXRD) is arguably the most definitive method for molecular structure determination, but it is often challenged by compounds that are liquids or oils at room temperature or do not form crystals adequate for analysis. Our laboratory previously reported a simple, cost-effective, single-step crystallization method based on guanidinium organosulfonate (GS) hydrogen bonded frameworks for structure determination of a wide range of encapsulated guest molecules, including assignment of the absolute configuration of chiral centers. Herein, we expand on those results and report a head-to-head comparison of the GS method with adamantoid “molecular chaperones”, which have been reported to be useful hosts for structure determination. Inclusion compounds limited to only two GS hosts are characterized by low R₁ values and Flack parameters, infrequent disorder of the host and guest, and manageable disorder when it does exist. The structures of some target molecules that were not included or resolved using the adamantoid chaperones were successfully included and resolved by the GS hosts, and vice versa. Of the 32 guests attempted by the GS method, 31 inclusion compounds afforded successful guest structure solutions, a 97% success rate. The GS hosts and adamantoid chaperones are complementary with respect to guest inclusion, arguing that both should be employed in the arsenal of methods for structure determination. Furthermore, the low cost of organosulfonate host components promises an accessible route to molecular structure determination for a wide range of users.