Date of Award

12-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Pharmaceutical Sciences

Track

Pharmaceutics

Research Advisor

Bernd Meibohm, Ph.D., FCP

Committee

Julian Hurdle, Ph.D. Richard E. Lee, Ph.D. P. David Rogers, Pharm.D., Ph.D. Charles Ryan Yates, Pharm.D., Ph.D.

Abstract

In an effort to combat the global Tuberculosis pandemic, Dr.Richard E. Lee and his group at St.Jude Children’s Research Hospital designed a novel series of anti-tuberculosis agents, spectinamides – semi-synthetic analogs of spectinomycin. Spectinamides are a potent inhibitor of mycobacterial ribosomes and overcome efflux mediated drug resistance in M. tb. Spectinamides have shown an excellent in vitro activity, which makes them well suited for further lead optimization and preclinical development. We hypothesized that through pharmacokinetic (PK) and pharmacodynamics (PD) model-based dosing optimization studies, we could strategically guide the selection and refinement of more potent and effective anti-TB spectinamides. Biopharmaceutical in vitro screening demonstrated that spectinamides in general have low plasma protein binding and are stable against hepatic microsomal metabolism. In vivo pharmacokinetic studies in rats revealed that the kidneys are the major route of elimination for spectinamides in their unchanged form. Radiolabeled biodistribution studies showed 84.7% of radioactivity accumulated 70% in urine, 12.6% in feces, and the remainder in the blood and other major organs. The unaccounted for residual 15.3% likely distributed into the epidermis and other surface tissue. In multiple-dose accumulation studies, the Cmax of radiolabeled compound after the 1st dose and the 8th dose of twice-daily dosing regimen was similar: 3.39µCi/mL and 3.55µCi/mL, suggesting no relevant accumulation of parent drug and metabolites. The concentration of radiolabeled compound was three times more in lungs and spleen as compared to whole blood, suggesting good tissue penetration. Macrophage uptake studies showed that Lee 1329, Lee1445 and Lee 1599 had significantly higher macrophage uptake than spectinomycin and streptomycin. Lee 1329 showed 6-fold and 2.2-fold higher uptake than streptomycin and spectinomycin, respectively. Based on the results of the in vitro experiments and preliminary PK/PD studies in rats, Lee 1599 was selected as the lead candidate compound. To predict PK/PD indices of antimicrobial efficacy, we performed model-based dosing optimization studies with Lee 1599. We used an in vitro PK/PD model system to simulate the rat PK conditions while evaluating antibacterial activities to predict effective dosing regimens for further in vivo efficacy studies. Our results have shown that Lee 1599 exhibits dose-dependent bactericidal effect. Lee 1599 showed up to 4-log reductions in bacterial counts at 100mg QD dosing. The PK/PD indices demonstrated that Lee 1599 elicits a concentration- and time-dependent killing with AUC/MIC as the optimal index. The model was put through numerical simulations to predict the effect of Lee 1599 in mice at various dosing regimens. The in vitro PK/PD simulated profile has suggested that high doses with frequent dosing intervals may demonstrate optimum in vivo efficacy. Consequently, we aimed to determine the pharmacodynamic interaction between Lee 1599 and existing anti-tuberculosis agent. We selected rifampicin as a model compound and applied a parametric approach to quantitatively assess the pharmacodynamic drug interaction between Lee 1599 and rifampicin. The three dimensional surface response assay demonstrated that there is an additive effect between both the agents as opposed to the conventional checkerboard assay, which suggested synergism between these agents. The results of surface response assay were validated using an in vitro PK/PD model for combination agents and in vivo efficacy trials, which showed an additive effect between Lee 1599 and rifampicin. Thus, quantitative assays such as the surface response assay seem to provide more reliable information on pharmacodynamic interactions as opposed to qualitative methods such as checkerboard assay. In conclusion, we have successfully supported the further development of spectinamides using a pharmacometric approach. We have identified a lead candidate compound Lee 1599 using an iterative PK/PD approach for its pre-clinical drug development. The application of PK/PD knowledge is essential for translating the in vitro screening assay findings to the in vivo stage, thus accelerating the drug development process. The results of the above studies can be used as a roadmap for the optimization of anti-infective agents in the early drug discovery and pre-clinical developmental phase.

DOI

10.21007/etd.cghs.2015.0323

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