A series of N(2),N(4)-disubstituted quinazoline-2,4-diamines has been synthesized and tested against multidrug resistant Staphylococcus aureus. A structure-activity and structure-property relationship study was conducted to identify new hit compounds. This study led to the identification of N(2),N(4)-disubstituted quinazoline-2,4-diamines with minimum inhibitory concentrations (MICs) in the low micromolar range in addition to favorable physicochemical properties. Testing of biological activity revealed limited potential for resistance to these agents, low toxicity, and highly effective in vivo activity, even with low dosing regimens. Collectively, these characteristics make this compound series a suitable platform for future development of antibacterial agents.

A series of N2,N4-disubstituted quinazoline-2,4-diamines has been synthesized and tested against Leishmania donovani and L. amazonensis intracellular amastigotes. A structure–activity and structure–property relationship study was conducted in part using the Topliss operational scheme to identify new lead compounds. This study led to the identification of quinazolines with EC50 values in the single digit micromolar or high nanomolar range in addition to favorable physicochemical properties. Quinazoline 23 also displayed efficacy in a murine model of visceral leishmaniasis, reducing liver parasitemia by 37% when given by the intraperitoneal route at 15 mg kg–1 day–1 for 5 consecutive days. Their antileishmanial efficacy, ease of synthesis, and favorable physicochemical properties make the N2,N4-disubstituted quinazoline-2,4-diamine compound series a suitable platform for future development of antileishmanial agents.

Visceral leishmaniasis is a neglected parasitic disease that has a high fatality rate in the absence of treatment. New drugs that are inexpensive, orally active, and effective could be useful tools in the fight against this disease. We previously showed that N2,N4-disubstituted quinazoline-2,4-diamines displayed low- to sub-micromolar potency against intracellular Leishmania, and lead compound N4-(furan-2-ylmethyl)-N2-isopropyl-7-methylquinazoline-2,4-diamine (4) exhibited modest efficacy in an acute murine model of visceral leishmaniasis. In the present work, thirty-one N2,N4-disubstituted quinazoline-2,4-diamines that had not previously been examined for their antileishmanial activity were evaluated for their potency and selectivity against Leishmania donovani, the causative parasite of visceral leishmaniasis. Quinazoline-2,4-diamines with aromatic substituents at both N2 and N4 exhibited potent in vitro antileishmanial activity but relatively low selectivity, while compounds substituted with small alkyl groups at either N2 or N4 generally showed lower antileishmanial potency but were less toxic to a murine macrophage cell line. Based on their in vitro antileishmanial potency, N4-benzyl-N2-(4-chlorobenzyl)quinazoline-2,4-diamine (15) and N2-benzyl-N4-isopropylquinazoline-2,4-diamine (40) were selected for in vivo evaluation of their pharmacokinetic and antileishmanial properties. While 15 displayed a longer plasma half-life and a greater area under the curve than 40, both compounds showed low efficacy in an acute murine visceral leishmaniasis model. Although the present study did not identify new quinazoline-2,4-diamines with promising in vivo efficacy, the reduced in vitro toxicity of derivatives bearing small alkyl groups at either N2 or N4 may provide clues for the design of safe and effective antileishmanial quinazolines.

We previously reported a series of N 2 ,N 4 -disubstituted quinazoline-2,4-diamines as dihydrofolate reductase inhibitors with potent in vitro and in vivo antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) strains. In this work, we extended our previous study to the Gram-negative pathogen Acinetobacter baumannii. We determined that optimized N 2 ,N 4 -disubstituted quinazoline-2,4-diamines are strongly antibacterial against multidrug-resistant A. baumannii strains when the 6-position is replaced with a halide or an alkyl substituent. Such agents display potent antibacterial activity, with MICs as low as 0.5 M, while proving to be strongly bactericidal. Interestingly, these compounds also possess the potential for antibiofilm activity, eradicating 90% of cells within a biofilm at or near MICs. Using serial passage assays, we observed a limited capacity for the development of resistance toward these molecules (4-fold increase in MIC) compared to existing folic acid synthesis inhibitors, such as trimethoprim (64-fold increase) and sulfamethoxazole (128-fold increase). We also identified limited toxicity toward human cells, with 50% lethal doses (LD 50 s) of Յ23 M for lead agents 4 and 5. Finally, we demonstrated that our lead agents have excellent in vivo efficacy, with lead agent 5 proving more efficacious than tigecycline in a murine model of A. baumannii infection (90% survival versus 66%), despite being used at a lower dose (2 versus 30 mg kg Ϫ1 ). Together, our results demonstrate that N 2 ,N 4 -disubstituted quinazoline-2,4-diamines have strong antimicrobial and antibiofilm activities against both Gram-positive organisms and Gram-negative pathogens, suggesting strong potential for their development as antibacterial agents.

Caspases are a family of cysteine-dependent proteases with important cellular functions in inflammation and apoptosis, while also implicated in human diseases. Classical chemical tools to study caspase functions lack selectivity for specific caspase family members due to highly conserved active sites and catalytic machinery. To overcome this limitation, we targeted a noncatalytic cysteine residue (C264) unique to caspase-6 (C6), an enigmatic and understudied caspase isoform. Starting from disulfide ligands identified in a cysteine trapping screen, we used a structureinformed covalent ligand design to produce potent, irreversible inhibitors (3a) and chemoproteomic probes (13-t) of C6 that exhibit unprecedented selectivity over other caspase family members and high proteome selectivity. This approach and the new tools described will enable rigorous interrogation of the role of caspase-6 in developmental biology and in inflammatory and neurodegenerative diseases.