Mitochondria are organelles present in eukaryotic cells that play a key role in regulating cells’ metabolic processes as well as cell death. The main function of mitochondria is to produce ATP, by oxidizing nutrients in a process called oxidative phosphorylation (OXPHOS). Besides this, mitochondria also play a critical role in calcium homeostasis, cell signaling, and apoptosis. Mitochondrial dysfunction is implicated in a plethora of diseases including neurodegenerative diseases, metabolic disorders as well as ageing and cancer. The triphenylphosphonium (TPP+) moiety has been used as a carrier to direct a wide variety of therapeutic and diagnostic cargo to mitochondria, in an effort to study and treat mitochondrial dysfunction. Studies in recent years show that TPP+ is not an inert carrier as previously thought. Many TPP+ conjugates have been shown to exert a negative effect on mitochondrial and cellular bioenergetics by decreasing the efficiency of OXPHOS. This phenomenon is called ‘mitochondrial uncoupling’. While mitochondrial uncoupling is undesirable for the TPP+ moiety to function as a carrier of cargo to mitochondria, controlled uncoupling has therapeutic applications in treatment of obesity and cancer. The extent of mitochondrial accumulation as well as potency of mitochondrial uncoupling caused by the TPP+ moiety increases with increasing length of the linker functionality in TPP+ conjugates. Most of the studies to date have focused on altering the linker length of the TPP+-linker-cargo conjugate to optimize the balance between safety and efficacy. However, very little is known about how structural modification of the TPP+ moiety itself affects mitochondrial uncoupling potency. Therefore, there is a need to understand the structure activity relationship (SAR) between modification of TPP+ structure and the effect of these structural changes on mitochondrial uncoupling and uptake. Towards this end, the first goal of this study was to understand the effect of modulating electron density on the phosphorus atom of TPP+ on the potency of uncoupling OXPHOS. Modifications to the TPP+ moiety included substitution of electron withdrawing and donating groups on the phenyl rings of TPP+, and replacing phenyl rings with bulkier napthyl rings. Modified TPP+ moieties were conjugated to five different linkers, which varied in length and lipophilicity, and the effect of these conjugates on mitochondrial bioenergetics was studied. The second goal of the study was to evaluate if the propensity of TPP+ to uncouple mitochondrial respiration can be modulated, independently of mitochondrial uptake. For this purpose, the uptake of modified TPP+-linker conjugates into isolated mitochondria and the uptake of fluorescently labeled modified TPP+-linker conjugates into mitochondria within whole cells was investigated. The ability of modified TPP+ to protect cells from oxidative stress by successfully delivering an anti-oxidant cargo to mitochondria within cells was also assessed. The results of these studies establish the first SAR for modulating TPP+ structure to either eliminate, optimize, or maximize uncoupling of mitochondrial OXPHOS, and led to identification of lead molecules for potential applications in the fields of mitochondrial delivery, anti-obesity therapy and anti-cancer therapy.

Melanoma incidence is increasing faster than any other cancer in the world today. Disease detected early can be cured by surgery, but once melanoma progresses to the metastatic stage it is lethal, with an overall median survival of less than one year. The poor prognosis for late stage melanoma patients is attributed to the intrinsic resistance of melanoma to all Federal Drug Administration approved melanoma therapies. Therefore, there is a critical need for novel treatment approaches that circumvent melanoma therapy resistance. Emerging evidence suggests that differences in melanoma metabolism relative to non-malignant cells represents a potential target for therapeutic intervention. The research presented here demonstrates the potential for using triphenylphosphonium-based compounds as a new therapeutic platform for metastatic melanoma that is designed to take advantage of these metabolic differences. In vitro experiments demonstrate that triphenylphosphonium-based compounds modified with an aliphatic side chain target melanoma cell mitochondria and promote melanoma cell death via mitochondria metabolism inhibition and subsequent reactive oxygen species production. Increased reactive oxygen species production results in decreased glutathione levels and an oxidized cellular state. There is also a structure-activity relationship between side chain length, metabolic disruption, and melanoma cell cytotoxicity. Further, results demonstrate that traditional in vivo triphenylphosphonium drug administration routes such as oral gavage, intraperitoneal injection, and intravenous injection do not result in significant tumor accumulation of triphenylphosphonium drugs. However, the use of a thermosensitive hydrogel delivery system localizes triphenylphosphonium drugs directly at the melanoma tumor site and decreases melanoma tumor growth rate. These results suggest that a hydrogel-based triphenlyphosphonium delivery system could potentially be a therapeutic strategy that circumvents melanoma resistance mechanisms in order to provide durable therapy for an increasing number of metastatic melanoma patients worldwide.