Zilebesiran, a ribonucleic acid interference agent targeting angiotensinogen, proves a promising approach in hypertension

Hypertension (HTN) is estimated to affect one in three adults worldwide, a prevalence that has doubled in the past 20 years and is expected to rise further.1 As the leading global risk factor for cardiovascular disease, HTN causes over 10 million deaths every year.2 Despite its consequences, HTN remains undertreated, with less than half of those treated exhibiting controlled blood pressure (BP).1 Existing treatments have limitations of efficacy, tolerability, and adherence.3 Indeed, only a worrying 50% of patients are still fully adherent to treatment after 12 months of treatment. One promising new approach is the use of ribonucleic acid (RNA) interference agents that target and knock down hepatic synthesis of angiotensinogen (AGT), the sole precursor of angiotensin peptides (Figure 1). The use of rapidly advancing technology has generated small interfering RNA (siRNA) with low immunogenicity linked to N-galactosidase (Gal-NAc).4 Gal-NAc siRNAs bind to a receptor (ASGPR) which is exclusively expressed in the liver and present at high abundance, which rapidly transports the siRNAs to endosomes. These serve as a depot for messenger RNA (mRNA) knockdown, to which siRNAs can recycle (Figure 1). Knockdown leads to reduced AGT generation4 and, as a consequence, renin-mediated angiotensin synthesis. This technique has already been successful therapeutically in rare diseases,4 as well as in hypercholesterolaemia (by targeting the liver protein PCSK-9).5 This figure shows the arrival of zilebesiran, a first-in-class AGT siRNA conjugated to triantennary Gal-NAc within the liver circulation followed by (1) high-level binding to the ASGPR; (2) clathrin-mediated endocytosis into endosomes; (3) recycling of ASGPR to the cell membrane and (4) release of AGT siRNA into the cytosol; (5) binding of AGT siRNA to RISC; (6) siRNA unwinding and strand separation; (7) discarding of the passenger strand and (8) binding of the guide strand to AGT mRNA; (9) cleavage of AGT mRNA into fragments; and (10) recycling of the AGT siRNA guide strand for further AGT mRNA cleavage. The endosomal reservoir of siRNA, together with siRNA recycling, likely accounts for the long duration of action in reducing AGT protein and hence reduction of angiotensin II levels and consequently blood pressure. AGT, angiotensinogen; ASGPR, asialoglycoprotein receptor; Gal-NAc, N-galactosidase; RISC, RNA-induced silencing complex. AGT siRNA is shown in green. AGT mRNA and fragments are shown in purple. The AGT siRNA, zilebesiran, is administered subcutaneously (sc) as a single dose and largely taken up by the liver, where it causes AGT knockdown (Figure 1). The main part (Part A) of Phase I studies6 used seven single ascending doses of zilebesiran (10, 25, 50, 100, 200, 400, and 800 mg), each with 12 untreated HTN subjects randomly assigned 2:1 to active or placebo treatment. Part B explored the effect of 800 mg zilebesiran sc under high- and low-salt dietary conditions, and Part E explored the effect of 800 mg zilebesiran sc when administered with irbesartan. The primary outcome was safety and tolerability, the secondary outcome was reduction in serum AGT, and the key exploratory outcome was change in BP measured by 24-h ambulatory BP monitoring (ABPM). Of the 107 patients enrolled, 5 had mild, transient injection site reactions, but there were no reports of hypotension, hyperkalaemia, or worsening of renal function resulting in medical intervention. Patients receiving zilebesiran had substantial dose-related decreases in serum AGT levels (≥90% by week 8 for doses ≥ 100 mg) that correlated with the administered dose. Single doses of zilebesiran (≥200 mg) were associated with decreases in systolic BP (SBP; >10 mm Hg) and diastolic BP (DBP; >5 mm Hg) by Week 8. These changes were consistent throughout the diurnal cycle and, at doses of ≥200 mg, sustained at 24 weeks. Results from Part B were consistent with a high-salt diet attenuating the effect on BP and Part E with an augmented effect on BP by co-administration with irbesartan.6 In summary, zilebesiran was not only well tolerated but also clearly produced substantial dose-dependent reductions in AGT. Additionally, using the gold standard of ABPM, zilebesiran reduced BP to a substantial and clinically relevant degree (∼8 and 12 mmHg at 8 and 24 weeks with the 800 mg dose vs. placebo, respectively). Interestingly, as shown in supplementary figure 5 the original phase I paper, substantial AGT knockdown seemed to be important for efficacy in BP reduction, consistent with the 50,000-fold higher levels of AGT in blood than those of renin.7 The KARDIA programme built on these exciting data. KARDIA-18 examined different dosing regimens of zilebesiran in untreated HTN patients, confirming the impressive reduction in SBP seen in Phase I, at ≥15 mmHg vs. placebo, with duration consistent with use of a 600 mg dose at 6-monthly intervals. There were no new safety signals that might hinder the development of the programme. KARDIA-2 examined the effect of combining 600 mg zilebesiran given with an angiotensin 1 receptor blocker (ARB; olmesartan), calcium antagonist (amlodipine), or diuretic (indapamide) in HTN. The top-line results of KARDIA-2 have recently been reported9 by the manufacturer, Alnylam, as showing clinically significant BP reductions when added to standard of care, and safety and tolerability data were reported as encouraging. The study was presented in April 2024 at the ACC meeting, and, compared with matched blinded placebo, zilebesiran reduced SBP by 14 mmHg in combination with indapamide, 10 mmHg in combination with amlodipine, and 4 mmHg in combination with olmesartan.10 The next step is initiation of KARDIA-3 (NCT06272487), which addresses the efficacy and safety of zilebesiran used as add-on therapy in a placebo-controlled trial in patients with HTN at high cardiovascular risk who are not controlled by standard of care medicines (on at least two but not more than four medicines). The primary outcome is change from baseline to 3 months in mean seated office SBP, with secondary measures which include ABPM at 3 and 6 months. Again, change in serum AGT will be explored. This programme, therefore, includes treatment-resistant HTN, but not restricted to this group. Based on what we currently know from KARDIA-1 and -2, the large and durable BP-lowering seen with zilebesiran in HTN in Phase 1 seems to be confirmed, at least out to 6 months. In addition, the studies with a Gal-NAc siRNA for hypercholesterolaemia, inclisiran, suggest that the general safety of the approach will be acceptable.5 However, there are still hurdles. In particular, the renal risk of combining two inhibitors of the renin–angiotensin system is well described.11 In contrast, the KARDIA programme explores the safety and additional efficacy of this combination, the results of which will be very interesting. In addition, there remains the issue of reversing the effects of zilebesiran during an episode of hypovolaemia (such as through dehydration, sepsis, or haemorrhage) to avoid acute kidney injury (AKI). This problem has not yet emerged but has been considered. Part B of the Phase 1 study5 showed that a high-salt diet could reverse BP-lowering, and intravenous (iv) saline might do this faster. In spontaneously hypertensive rats,12 BP could be rapidly reversed by iv administration of angiotensin II or noradrenaline and partially restored by fludrocortisone. If sustained reversal might be required, a specific REVERSIR™13 has been designed as a short, synthetic, high-affinity oligonucleotide that can reverse the siRNA’s effect substantially over a few days. Whether this approach will be needed is currently unclear, but AKI likely remains the major risk of using an AGT siRNA in HTN. Of course, studies of safety and clinical effectiveness in broader populations, including patients with co-morbidities, are still awaited, as are cardiovascular outcome studies. The cost-effectiveness of this approach will depend on pricing (as existing treatments are all relatively cheap). Finally, if shown to be safe and acceptable to patients, the AGT siRNA zilebesiran stands out as a promising approach to the combined problems of treatment-to-target and of non-adherence in HTN treatment (a long-standing and substantial problem in routine care), because of the large size and durability of its BP-lowering effect. No funding was used in completing this work. No new data were generated or analysed in support of this research. Biography: David J. Webb is Emeritus Professor of Therapeutics and Clinical Pharmacology at the University of Edinburgh (https://en.wikipedia.org/wiki/David_Webb_(pharmacologist)). He led the Department of Medical Sciences in Edinburgh before establishing and leading the University’s Centre for Cardiovascular Science, with Wellcome Trust support, while also directing the University’s Clinical Research Centre and research education programme. His research focuses on arterial structure and function, and on the pathophysiology of hypertension, hypertensive renal disease, endothelial dysfunction, and arterial stiffness, with a particular interest in the pharmacology and therapeutic potential of the endothelin and renin–angiotensin systems. Having undertaken both preclinical and early clinical studies, alongside mechanistic research, he was involved in the first-in-human studies with a renin inhibitor and with a small interfering RNA against angiotensinogen, as well as some of the earliest clinical studies with endothelin receptor antagonists. He has a current h-index of >100, with over 700 papers and 5 books in the cardiovascular field. Professor Webb is a Fellow of the Royal Society of Edinburgh, the UK Academy of Medical Sciences and British and Irish Hypertension Society, and the American Heart Association and European Societies of Cardiology and Hypertension. He was awarded the SKB and Lilly Prizes for clinical pharmacology by the British Pharmacological Society and was winner of the Tomoh Masaki Award for endothelin research. He was the former president of the British Pharmacological Society (BPS), honorary president of the European Association for Clinical Pharmacology and Therapeutics (EACPT), and recent chair of the Clinical Division of the International Union of Basic and Clinical Pharmacology (IUPHAR), for whom he ran their 19th World Congress for Basic and Clinical Pharmacology at the Scottish Event Campus in Glasgow in 2023. In 2020, he was appointed Commander of the British Empire (CBE) in the Queen’s Birthday Honours for his research in the field of clinical pharmacology, largely in work on hypertension and cardiovascular disease.