Development of secretory phospholipase A2 responsive liposomes and tracking synthetic novel lipids in biological samples
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Lipid based nanoparticulate drug-carriers, such as long-circulating sterically-stabilized liposomes (SSL), can alter a drugs pharmacokinetic profile and improve its antitumor activity. However, mechanisms to “tune” their drug release kinetics and ability to track drug-carrier disposition in vivo are limited. The goals of this dissertation were to develop lipid based nanoparticulate drug carriers that are responsive to elevated expression of secretory phospholipase A2 (sPLA2) in malignant tissues, to modulate drug release and track the distribution and metabolism of these formulations from biological samples. In Chapter 1 we reviewed lipid-based nanoparticulate drug-carriers and contemporary approaches to modulate in vivo drug release. In Chapter 2 we developed an electrospray ionization-mass spectrometry (ESI-MS) method to determine sPLA2 selectivity on individual and combinations of lipids. Studies in Chapters 2 and 3 demonstrated that in the presence of sPLA2, incorporation of lipids like distearoylphosphatidylglycerol (DSPG), distearoylphosphatidylethanolamine (DSPE) and synthetic odd-chain (C:15:0/C:16:0) lipids: 1-O-hexadecyl-2-pentadenoyl-sn-glycerol-3-phosphocholine (C31PC) and 1-O-hexadecyl-2-pentadenoyl-sn-glycerol-3-phosphomethanol (C31PM) in drug carriers could significantly change the release profiles of 6-carboxyfluoresein (6-CF, a fluorescent probe). These studies supported our hypothesis that sPLA2–mediated drug release could be modulated by altering lipids’ acyl chain length and use of head groups with different physicochemical properties. In Chapters 3 and 4, we demonstrated that odd chain acyl lipids, i.e., C31PC, could be used to track the deposition of liposomes in vivo. Interference of endogenous lipids in biological samples to C31PC was negligible. C31PC was extracted from biological samples and identified using ESI-MS. This method may be used to gain mechanistic insight into the disposition, degradation and release kinetics of lipid-nanoparticles. In Chapter 4 we also demonstrated that acidification of samples during a Bligh and Dyer extraction improved the extraction efficiency of anionic lipids. This dissertation reports a deviation from the status quo where the focus is on encapsulating the greatest quantity of drug stably and using external physiological stimuli to trigger “burst” release from particles. Development of sPLA2-targeted formulations by incorporating sPLA2-preferred lipids has the potential to enhance drug carrier deposition in tumors, optimize exposure profiles to maximize antitumor activity and may be extended to additional drugs, targeting ligands or contrast agents.