The role of cell-surface peptides in the regulation of GPCR signalling is well documented. These peptidases regulate levels of circulating bioactive peptides, which function to initiate GPCR-mediated signalling. ACE compound peptidase (EC 220.127.116.11) is a zinc-dependent metallopeptidase that converts the inactive angiotensin I to angiotensin II by releasing the C-terminal dipeptide, His9-Leu10 (Figure 1A). Angiotensin II is a vasoconstrictor and exerts its effect through two types of angiotensin II receptors, AT1 and AT2. The AT1 receptor mediates most of the physiological and pathophysiological actions of angiotensin II and is the predominant receptor subtype expressed in the cardiovascular system. Interaction of the AT1 receptor with angiotensin II activates Gq/11, Gi, G12 and G13 proteins, leading to the mobilization of intracellular calcium, generation of reactive oxygen species and activation of numerous PKs and mitogenic signalling pathways [reviewed in Mogi et al. (2009)]. AT2 receptors couple to G proteins to activate PLC, promoting the mobilization of intracellular calcium and activation of PKC [reviewed in Porrello et al. (2009)]. Angiotensin II causes a plethora of effects, including tissue remodelling, leukocyte infiltration, inflammation, atherosclerosis, endothelial dysfunction, myocardial infarction, stroke, and heart and renal failure [reviewed in Cheng et al. (2005)]. Similarly, endothelin-converting enzyme-1 (ECE-1, EC 18.104.22.168) is responsible for the production of the vasoconstrictor endothelin-1 from big endothelin. Endothelin-1 can activate both endothelin A (ETA) and ETB receptors to elicit a broad range of signalling responses [reviewed in Khimji and Rockey (2010)]. Activation of ETA receptors by endothelin-1 promotes mobilization of intracellular calcium, indicating that ETA receptors are coupled to Gq/11 proteins. ETA receptors primarily mediate the vasoactive and proliferative effects of endothelin-1. ETB receptors have been proposed to act as endothelin-1 scavengers and thus, reduce circulating endothelin-1 levels. Thus, proteolysis acts to promote activation of GPCRs. In contrast, proteolysis can also act to prevent activation of GPCRs. For example, neprilysin (NEP, neutral endopeptidase 24.11, EC 22.214.171.124) cleaves and inactivates both bradykinin (Gly4Phe5 and Pro7Phe8) and substance P (SP, Gln6PhePhe-GlyLeu10) (Matsas et al., 1984), thus preventing activation of their respective GPCRs, the bradykinin 2 receptor (B2 receptor) and the neurokinin 1 receptor (NK1 receptor) (Figure 1A). Peptidases can therefore play a major role in the production of vasoactive peptides and, therefore, regulate vascular functions and dysregulation can lead to vascular diseases. A great deal of time and effort has been spent on the development of peptidase inhibitors for the treatment of hypertension. It is over 30 years since the first ACE inhibitor, captopril, was designed (Ondetti et al., 1977). Now, however, other ACE inhibitors with improved pharmacokinetics and pharmacodynamics have since been developed and include enalaprilat (MK-421) (Gross et al., 1981) and imidaprilat (Ikeo et al., 1992). ACE inhibitors are effective treatments for hypertension and congestive heart failure (CONSENSUS, 1987; SOLVD, 1991) and are also beneficial for patients with atherosclerosis (Yusuf et al., 2000). With the success of ACE inhibitors for the treatment of hypertension, it was thought that the development of compounds able to inhibit multiple peptidases, thereby potentiating the effects of dilator peptides such as bradykinin, while reducing the availability of constrictors such as angiotensin II, may offer an even better way of treating diseases such as hypertension. Indeed, dual or triple vasoactive peptidase inhibitors have been developed. These compounds inhibit the proteolytic activities of ACE, ECE-1 and NEP in various combinations. The first described dual inhibitors were alatriopril and glycoprilat. Both compounds inhibit the activities of both ACE and NEP (Gros et al., 1991). Alatriopril was shown to be more effective than captopril alone in reducing cardiac hypertrophy in rats with myocardial infarction (Bralet et al., 1994). Later, omapatrilat (BMS-186716) was developed and was shown to be more effective in reducing blood pressure in humans than either placebo or ACE inhibitors (Neal et al., 2002; Regamey et al., 2002). However, omapatrilat failed in phase III clinical trials and was discontinued due to an increased incidence of angioedema as an unwanted side effect. CGS 35601, a triple vasopeptidase inhibitor, prevents the activities of ACE, NEP and ECE-1 (Trapani et al., 2004) and significantly reduced both systolic and diastolic blood pressure in a number of preclinical rodent models of hypertension (Daull et al., 2005; 2006a). Further, a preclinical safety profile assessment of CGS 35601 showed it to have no effect on either hepatic or liver toxicities (Daull et al., 2006b). Although these triple inhibitors may represent the future of peptidase inhibitors for the treatment of disease (Table 1), no clinical trials using triple peptidase inhibitors have yet been conducted. So, although these dual and triple peptidase inhibitor compounds are promising in humans and animal models of hypertension, none have yet been approved for the treatment of human disease. Thus, ACE inhibitors remain the compounds of choice for the treatment of hypertension, often in combination with angiotensin or β-adrenoceptor antagonists or diuretics. NEP also plays a major role in the catabolism of endogenous opioid peptides such as the enkephalins and dynorphins. Thus, together with aminopeptidase N (EC 126.96.36.199), NEP represents a major target for the development of drugs for the treatment of acute and chronic pain [reviewed in Thanawala et al. (2008)].