Novel fluorescent angiotensin AT1 receptor antagonists

© 2012 Dr. Marilena A. GiarrussoHypertension is a severe condition that affects many people worldwide. The pathophysiology of hypertension is unknown. Sartans (selective AT1 receptor antagonists) are known to be the most direct and widely used class of antihypertensive drugs that block the vasoconstrictive hormone angiotensin II from binding at its AT1 receptor. Since the discovery of Losartan (2), many drugs are clinically used today. Sartans display a diverse pattern of antagonism however there is some debate as to why this occurs. Some believe it’s due to the antagonist induced changes in the AT1 receptor conformation, others believe it’s due to the levels of cell surface receptor expression and internalisation of the antagonist-receptor complex. Binding of fluorescent sartans to AT1 receptors, will enable the investigation of AT1 receptor localisation. This will provide an insight of the molecular pharmacology of AT1 receptors in cell and tissue systems and in turn will provide a better understanding of the physiological mechanisms involved with the disease. With the aim of preparing fluorescent sartans for use in the study of the molecular pharmacology and cellular localisation of AT1 receptors in cell-based systems, initial work was thus directed towards the synthesis of a series of analogues 103 - 114 of Fonsartan (14), in order to explore the effect of heteroatom substitution and substituent size on the AT1 receptor binding ability of the sartans. The approach involved direct coupling of the common bromo-biphenyl scaffold 32 with several novel aryl-thio substituted imidazoles 90 - 95. Also prepared were coumarin analogues 151 and 152 of Fonsartan, which required synthesis of the novel imidazole 149. In addition, the coumarin analogue 176 of Losartan (2) was also synthesised. Pharmacological testing revealed that compounds 103 – 108, 109 – 114, 151 and 152 were potent AT1 receptor antagonists. The novel AT1 receptor antagonists synthesised in this project that have shown strong inhibition activity and possess fluorescence emission properties suitable for in vitro cellular imaging were sartans 106, 112 - 114, 151 and 152. The novel fluorescent angiotensin AT1 receptor antagonists 113, 114 and 176 showed weak inhibition activity. Results from the in vitro cellular imaging, using Chinese hamster ovary (CHO) cells stably expressing the rat AT1a receptor, revealed that the novel fluorescent sartans 106, 112 - 114, 151 and 152 were too lipophilic to observe the localisation of AT1 receptors in CHO cells. The synthesis of less lipid soluble selective AT1 receptor antagonists with appropriate fluorescence emissions useful for biological investigations may prove as useful tools for the investigation of cellular localisation and trafficking of the AT1 receptors both in vitro and in vivo