Antibodies as specific renin inhibitors: studies with polyclonal and monoclonal antibodies and Fab fragments

Exploring the substructural space of indole-3-carboxamide derivatives binding to renin: a novel active-site spatial partitioning approach

Renin has recently attracted much attention in the antihypertensive community, since this enzyme starts the angiotensin-converting cascade and forms the rate-limiting step in this cascade. In the present study, we describe a new method called active-site spatial partitioning (ASSP) for quantitatively characterizing the nonbonding interaction profile between renin and the substructures of indole-3-carboxamide derivatives-a novel class of achiral renin inhibitors that exhibit both high affinity and strong specificity for renin, thus blocking its active state-on the basis of structural models of protein-ligand complexes. It is shown that the ASSP-derived potential parameters are highly correlated with the experimentally measured activities of indole-3-carboxamides; the statistical models linking the parameters and activities using a sophisticated partial least squares regression technique show much promise as an effective and powerful tool for generalizing and predicting the pharmaceutical potencies and the physicochemical properties of other modified derivatives. Furthermore, by visually examining substructure-color plots generated by the ASSP procedure, it is found that the relative importance of nonbonding contributions to the recognition and binding of a ligand by renin is as follows: steric < hydrophobic < electrostatic. The polar and charged moieties that float on the surface of the ligand molecule play a critical role in conferring electrostatic stability and specificity to renin-ligand complexes, whereas the aromatic rings embedded in the core region of the ligand are the main source of hydrophobic and steric potentials that lead to substantial stabilization of the complex architecture.

Renin Inhibition: What Are the Therapeutic Opportunities?

Blockade of the renin-angiotensin system with angiotensin-converting enzyme inhibitors or angiotensin receptor blockers has become a crucial element in cardiovascular and renal medicine. This review evaluates the potential of renin inhibition as an adjunct to therapies that depend on renin system interruption.

Is there a future for renin inhibitors?

Pharmacological interruption of the renin-angiotensin system is possible at three major sites, the angiotensin-converting enzyme (ACE), the AT1 receptor and at the interaction of renin with its substrate, angiotensinogen. Skeggs and his associates in 1957 argued logically but without prognostic accuracy that 'since renin is the initial and rate-limiting substance in the renin-angiotensin system, it would seem that the renin inhibition approach would be the most likely to succeed'. In fact, the development of agents that act at all three levels has enjoyed substantial success, yet renin inhibition, which showed early progress in studies in humans, has languished. Our task in this essay is to review the reasons for the slow evolution of renin inhibition and to discuss the potential of such agents in modern pharmacotherapy. All of the structure-action relationships have involved variation on the original peptide structure. The possibility that alternative approaches based on x-ray crystallography and reconstruction of the structure of the active site would lead to novel agents, appears not to have been explored systematically. This opportunity is all the more attractive because renin is one of the few targets that is actually soluble and amenable to x-ray crystallographic studies. At the moment, it appears that all renin inhibitor development programs have been closed, although hints periodically reappear to indicate that one company or another is pursuing a novel agent. The decision to close programs seems to have reflected not the therapeutic potential of renin inhibitors, but rather the cost of their synthesis, continuing problems with bioavailability and the remarkable success of the competitor class--the AngII antagonists. We believe that the potential of renin inhibition in human therapy has been under estimated and still shows substantial promise.

Tissue angiotensin generation and regulation of vascular tone

The renin-angiotensin system is intimately involved in the control of sodium and water balance, the activity of the sympathetic nervous system, mitogenesis and the regulation of vascular tone. There is evidence that many of these effects may be controlled at a local level by independent tissue renin-angiotensin systems. Drugs that are specific inhibitors of the cascade have proved powerful tools for dissecting the physiology of the renin-angiotensin system, and are of major benefit in the treatment of hypertension and chronic heart failure. Recent evidence suggests that variations in the genes coding for components of the system may affect the risk of developing hypertension and ischaemic heart disease.

Pharmacology of renin inhibitors and their application to the treatment of hypertension

Several different strategies have been followed to block the activity of renin, the enzyme catalysing the first and rate-limiting step in the renin-angiotensin cascade. The unique substrate specificity of this enzyme makes it an attractive target for specifically interfering with the renin-angiotensin system. Attempts to block the activity of renin in animals by an immunological approach, with either active or passive immunization against renin, have been successful. This approach has not been considered as a realistic therapy in humans for the treatment of hypertension or heart failure, but has provided useful tools for purifying and quantifying renin. Considerable efforts have been focused on the design of orally active, synthetic inhibitors of renin. This has resulted in the discovery of low molecular weight pseudo-tetrapeptide compounds that are resistant to enzymatic cleavage and are potent and selective inhibitors of renin. Studies in animal models and preliminary studies in humans indicate that renin inhibitors have the same therapeutic potential as angiotensin-converting enzyme inhibitors. However, the generally poor oral bioavailability and rapid elimination of currently available renin inhibitors have prevented their development as useful drugs. Inhibitors with better oral bioavailability and a long duration of action are needed to assess their full therapeutic potential and to determine whether they offer advantages over the angiotensin-converting enzyme inhibitors or the more recently developed angiotensin II-receptor antagonists.

Effects of chronic administration of a monoclonal antibody against human renin in the marmoset

In this study, the hypotensive efficacy of R-3-36-16, a monoclonal antibody against human kidney renin, was investigated during chronic administration to a primate. R-3-36-16 was given by continuous intraperitoneal infusion with osmotic minipumps to normotensive marmosets fed a low-sodium diet in doses of 30 or 300 micrograms/kg/day for 14 days. The lower dose had no effect on blood pressure (BP) or plasma renin activity (PRA). After two days of treatment, the higher dose reduced PRA by 57% and lowered BP by 13 +/- 7 mm Hg. Although the hypotensive response persisted after 14 days of treatment (-17 +/- 2 mm Hg), PRA had recovered to pretreatment levels. BP gradually returned to pretreatment values in the week after stopping the treatment. There was no evidence of an immune reaction when an acute challenge dose of R-3-36-16 was given 7 weeks after stopping the chronic treatment. Thus, R-3-36-16 appears to be an effective and well-tolerated hypotensive agent during chronic administration to sodium-depleted primates. The hypotensive response does not seem to be directly related to the inhibition of renin in the plasma.

Monoclonal antibodies to human renin: properties and applications

A series of 11 different monoclonal antibodies generated against human kidney renin have been characterised. Their binding affinity, inhibition of renin activity, epitope distribution, crossreactivity with related enzymes and finally in vivo pharmacological effects were analysed. All antibodies were found to be specific for primate renin recognising 6 independent antigenic structures on the renin molecule. They expressed different effects on renin activity namely (1) no inhibition, (2) only partial, or (3) complete inhibition. Partially inhibiting antibodies demonstrated specific degrees of inhibition (30, 60 or 80%). One antibody, R-36-16, demonstrated an IC 50 of 1.3 X 10(-11) M/L and, when injected into marmosets, induced complete inhibition of plasma renin activity and reduction of blood pressure. Using a selected pair of antibodies a radioimmunoassay has been established providing a fast and highly reproducible determination of human and marmoset immunoreactive renin, detecting both active and inactive renin down to concentrations of 10 pg/ml (1.25 X 10(-17) moles of renin per 50 microliter sample).

Modification of the interaction of human renin with different substrates by monoclonal antibodies

The mechanism by which anti-renin antibody inhibits renin activity was studied by following the kinetics of the reaction with angiotensinogen or a low molecular weight synthetic substrate, tetradecapeptide (TDP). Two monoclonal antibodies (70 pM) inhibited the production of angiotensin I from angiotensinogen but they differed when hog TDP was used as a substrate. R3-47-10 partially and non-competitively inhibited, whereas R3-36-16 stimulated the activity of renin. This is in contrast to the effects of the synthetic renin inhibitor, CGP 29 287, which competitively inhibits the enzyme activity with both substrates. These antibodies probably bind to the renin molecule on the flap which protects the active cleft. Angiotensinogen may be prevented from entering the cleft due to steric hindrance from bound antibody. However TDP, because of its smaller size may still be able to reach the catalytic site. In addition R3-36-16 might freeze the flap in an open position allowing a greater turnover of TDP whereas R3-47-10 may prevent the flap from fully opening and thereby hinder the reaction of TDP with the active site.

Human renin inhibiting dipeptide

KRI-1177, a dipeptide containing nor-statine inhibited renin activity in human and Japanese monkey plasma to a markedly greater extent than that in dog, rabbit and rat plasma. The systemic blood pressure of anesthetized monkeys was lowered by intravenous injections of this compound which also reduced plasma renin activity and concentration of angiotensins. KRI-1177 appears to selectively inhibit primate renin activity, thereby producing hypotension.

Defining the physiologic and pathophysiologic roles of renin: the role of specific inhibitors

Although renin was identified as playing a role in cardiovascular homeostasis by the experiments of Goldblatt in the 1930's, neither its physiologic role in organs other than the kidney nor its contribution to the genesis of essential hypertension has been defined as yet. It is difficult to interpret studies with converting enzyme inhibitors because of their multiple pharmacologic effects. Specific inhibitors of renin appropriate for clinical investigation would help resolve many questions. Four classes of compounds have been demonstrated to be renin inhibitors of high potency: specific antibody, general peptide inhibitors of acid proteases, analogs of angiotensinogens, and peptides that are related to the amino-terminal sequence of prorenin. Of these, it is likely that angiotensinogen analogs will be the first applied in human studies. The minimal substrate for renin has the sequence: His-Pro-Phe-His-Leu-Leu-Val-Tyr. Variants of this sequence have yielded competitive inhibitors. Recently, remarkably active compounds have been synthesized by reducing the peptide bond that is cleaved by renin, or by incorporating the amino acid statine, found in pepstatin. These compounds have been shown now to be effective in dogs, rats, and monkeys, and most recently, preliminary studies have reported their efficacy in humans. Recent studies with one of these inhibitors, RIP, raise questions concerning both its specificity and site of action.

Construction of a model for the three-dimensional structure of human renal renin

The aspartyl proteases that have had their complete three-dimensional structures determined by x-ray diffraction techniques exhibit a high degree of structural homology and a correspondingly high degree of sequence homology. Using this homology, we constructed a model for the three-dimensional structure of human renal renin. We then refined and evaluated the model with the energy refinement program called CHARMM. We found that the model for human renin differs from that of mouse submaxillary gland renin in certain features, which may account for the differences in substrate specificity and antibody binding. Amino acid differences between human and mouse renin in the regions that bind the P1' side chain of the substrate appear to change only the shape of the S1' subsite of the enzyme, so that either valine or leucine side chains of the substrate can be accommodated by human renin. Amino acids in the solvent-accessible surface of the 75-85 flap appear to be distinctly different between the two structures and could account for the differences observed in antibody binding to human and mouse renin.

New monoclonal antibodies directed against human renin. Powerful tools for the investigation of the renin system

Monoclonal antibodies directed against human renin were obtained by the fusing of myeloma cells with spleen cells from Balb/c or high-responder Biozzi mice injected with pure tumoral or highly purified renal renin. These procedures resulted in the production of seven stable monoclonal antibodies to human renin. Antibodies in the hybridoma culture medium were screened by binding to pure iodinated renin or insolubilized renin in a solid phase assay. The concentration of purified antibodies that provided a 50% binding to iodinated renin varied from 1 X 10(-10) to 1 X 10(-7) M. Two monoclonal antibodies were found to be potent inhibitors of renin enzymatic activity in vitro, behaving as noncompetitive inhibitors (Ki, 1 to 4 X 10(-10) M). They were specific for primate renin. Three monoclonal antibodies provided suitable immunoadsorbants for renin purification. One of these immunoadsorbants was used for large-scale purification of the renal enzyme, resulting in an 825-fold renin enrichment in a single step. Two antibodies were able to distinguish between active and inactive renin and enabled concomitant separation and purification of the two enzyme forms in various biological fluids. Monoclonal antibodies also stained human and monkey renal renin when indirect immunofluorescence and peroxidase-antiperoxidase techniques were used. A highly sensitive radioimmunometric assay of renin was constructed with two monoclonal antibodies. The sensitivity of this improved assay should permit the detection of renin in normal human plasma. Monoclonal antibodies have been shown to be superior to polyclonal antibodies in the following areas: the separation of active from inactive renin, the purification of renin from biological fluids, and the setting up of a direct assay of plasma renin.