Drug discovery with phenacylhomoserine lactones
Oliver, Colin Matthew
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The first step in drug discovery involves identification of molecules of interest that originate from either natural or synthetic chemical sources. Natural products with desired biological properties or targets are used as templates for a myriad of structural modifications designed to generate libraries of new compounds. Biological screening and SAR analyses allow for determination of structural features beneficial/detrimental to next generation drugs and identification of lead compounds with better specificity. AHLs are methionine-derived signal molecules produced by gram negative bacteria that are used to control expression of population-sensitive phenomena (biofilm formation, resistance, etc) through a process known as QS. The canonical AHL, 3-oxo-dodecanoyl- L-homoserine lactone (OdDHL), is produced by P. aeruginosa. OdDHL displays anti-cancer activity and is being explored as a novel therapy for prostate cancer. While promising, the QS signaling effects of OdDHL can be lethal to a small percentage (5%) of immune-compromised cancer patients who become highly susceptible to nosocomial P. aeruginosa infections. We have designed and synthesized a library of OdDHL analogs characterized by the addition of a phenyl substituent. By varying structural characteristics and conducting SAR analyses, we have been able to determine structural elements required for both anti-cancer and QS signaling activities. Furthermore, we have identified a lead compound, 3-oxo-12-phenyldodecanoyl-L-homoserine lactone ( 12b ) that maintains cancer cell growth inhibitory activity while minimizing QS activation. Upon identification, lead compound 12b was used to explore potential mechanisms of cancer growth inhibition in comparison to control structures. 12b is capable of inhibiting the growth of a variety of cancer cell lines without directly disrupting attachment. PC3 prostate cancer cells are most susceptible to AHL-mediated growth inhibition. Yet, LNCaP prostate cancer cells are less sensitive, eliminating the hypothesis that sensitivity to AHLs may be tissue related. Analysis of proteomic differences between the two prostate cell lines identified PSMA as a protein whose expression negatively correlates with AHL sensitivity. From this, we hypothesized that PSMA expression disrupts AHL-mediated growth inhibition. Generation of PC3 cells expressing various levels of PSMA demonstrated that PSMA expression introduces 'resistance' to AHL-mediated growth inhibition. Induced resistance to AHLs is unrelated to PSMA enzymatic activity as inhibition of enzyme function with two distinct inhibitors had no effect on AHL activity. Knockdown of PSMA expression with shRNA reversed the induced resistance, confirming that PSMA expression is sufficient to disrupt AHL-mediated growth inhibition. Identification of second generation AHLs that maintain "anti-cancer" activity, yet minimize QS signaling activity is important to developing this novel avenue for cancer chemotherapy. In prostate cancer, PSMA is overexpressed. Therefore, determination of the role of PSMA as a modulator of AHL activity is essential to the success of this therapeutic opportunity.