Abstract: “Multi-drug resistant bacteria are a major threat to human health worldwide. Specifically, methicillin-resistant S. aureus (MRSA) was reported as a high priority pathogen by the World Health Organization in 2017. As a result, classes of antibiotics with efficacy against these problematic Gram-positive strains are urgently needed. Fusidic acid is a gram-positive-only antibiotic that has been used to treat MRSA infections. Although fusidic acid has been efficacious in the clinic since its introduction in the 1960s, it requires a high dosing regimen due to its high resistance frequency. Herein, we report a structural-activity relationship campaign resulting in the synthesis of the first equipotent derivatives of fusidic acid. Our derivatives show potent whole cell activity and an improved resistance profile. Lastly, our lead derivative displays improved efficacy in vivo against a fusidic acid resistant strain.”
East Central IL ACS Undergraduate Research Conference
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Nice work! Good Presentation!
I know your work was on the synthesis of the derivatives based on SAR, Did you use computational programs to narrow down on the derivatives that you synthesised. Did you individually chose the molecules or were these a subset of a priorcombinatorial library?
Also I am curious to know how the frequency resistance was determined and if you could explain briefly it will be appreciated!
Thank You!
Thank you so much – these are excellent questions! I appreciate you taking the time to watch my presentation.
To determine which derivatives to synthesize, we first validated the existing SAR of fusidic acid. Upon modulating the carboxylic acid, the acetate, and the two alcohol groups, we saw at least some reduction in the activity against S. aureus. The activity against S. aureus was determined by calculating the MIC. These results align with previously determined results. We also docked fusidic acid to its target protein, and we saw that the side chain of fusidic acid binds to a relatively large hydrophobic binding pocket. With all of this information in mind, we chose to target the fusidic acid side chain when making derivatives, in hopes to increase the affinity of fusidic acid to its target protein. We found that the rotational restriction conferred by the alkene in the side chain is critical for potent activity. We also found that the two side chain methyl groups were critical for potent activity. Unfortunately, the side chain methyl groups represent a site of oxidation in vivo. Thus, we synthesized the cyclic derivatives and the halogenated derivatives to maintain the symmetry of the side chain while also removing that site of potential oxidation. I hope this answers your first question!
As for how the resistance frequency was determined, some bacterial cells were grown on agar plates without antibiotic present, while other bacterial cells were grown on agar plates with a concentration of antibiotic at either 2x, 4x, 8x, 16x, or 32x, the MIC. Resistant colonies were generated. To find the resistance frequency, the number of resistant colonies was divided by the number of colonies grown on the equivalent plate without an antibiotic present.
Very sharp poster and good deal of work! Might I ask why you needed to protect the acid over the alcohol? Also do you have a good rationale for why the cross metatheses yields were so different between the cPent and cHex derivatives? Finally, what do you think is behind the cPent being more metabolically stable than the cHex – this seems like a minor change to me?
Thank you
Excellent questions! Thank you so much for listening to my presentation.
In regards to your first question, the two alcohol groups are both rather sterically hindered, so it is unlikely they will participate in any undesirable reactions.
For your second question, I would hypothesize that the cross-metatheses yields differ between the two reactions due to the cyclohexyl derivative being more difficult to extract from the crude mixture than the cyclopentyl derivative.
In regards to your last question, I would hypothesize that the increased hydrophobicity and steric hindrance of the cyclohexyl derivative relative to the cyclopentyl derivative would account for the difference in metabolic stability. Potentially this difference in hydrophobicity and steric hindrance translates to a difference in protein binding in vivo.