First in vivo base-editing trial shows promise

Since the first report of CRISPR-Cas9 gene editing in human cells in 2013,^1,2 CRISPR genome editing has emerged as an exciting therapeutic platform for numerous human diseases. The approval of the world’s first CRISPR-Cas9 gene editing therapy, CASGEVY, by the UK’s regulator on November 16, 2023 and the US Food and Drug Administration (FDA) on December 8, 2023, was a historic milestone. CASGEVY is developed by Vertex Pharmaceuticals and CRISPR Therapeutics for the treatment of sickle cell disease and β-thalassemia. Through editing BCL11A in hematopoietic stem cells (HPSCs) isolated from the patient’s bone marrow via electroporation of a synthetic guide RNA (gRNA) and Streptococcus pyogenes Cas9 protein in the lab, CASGEVY enhances the expression of a fetal form of hemoglobin. The edited HPSCs are then transfused back to patients. The landmark approval of CASGEVY heralds a new era of medicines.

Pioneered by Dr. David R. Liu at Harvard University, base editing induces more precise base conversion without creating double-stranded DNA breaks, unlike parental Cas9 gene editing.^3,4 A leading base-editing therapy drug for cardiovascular disease (CVD) developed by Verve Therapeutics (VERV-101) entered clinical trials for blood lipid control in New Zealand in 2022.⁵ Elevated circulating levels of low-density lipoprotein cholesterol (LDL-C) are a key risk factor for CVD. Recent therapeutic advances in lowering cholesterol considerably reduced the incidence of CVD. However, current medicines require daily oral pills or frequent injections. A one-time solution offered by permanent gene editing could be a game changer for CVD prevention.

Unlike blood disorders, CVD and many other conditions are less amenable to ex vivo gene editing followed by transplantation. However, direct in vivo gene editing of a patient’s DNA in specific tissues and organs is more challenging, requiring efficient vectors for targeted delivery. Several recently approved gene replacement therapies employ adeno-associated virus (AAV) for in vivo delivery. However, the high costs and the AAV-associated immunotoxicity when used in high doses pose additional challenges for AAV gene therapy.⁶ Moreover, the long-lasting expression of therapeutic transgenes delivered via AAV is undesired for gene-editing therapies, as it would potentially lead to the accumulation of unwanted off-target editing events. VERVE-101 utilizes lipid nanoparticles (LNPs), like those used in the mRNA COVID-19 vaccines,⁷ to deliver the base editor mRNA and the gRNA designed to inactivate the gene encoding proprotein convertase subtilisin/kexin type 9 (PCSK9) by disrupting the PCSK9 splice donor site. PCSK9 is a major degrader of the LDL receptor (LDLR), a key molecule for the removal of LDL from the plasma. Naturally occurring loss-of-function mutations in PCSK9 are associated with lower plasma levels of LDL-C and reduced risk of CVD.^8,9 Therefore, inhibition or genetic inactivation of PCSK9 has been actively pursued as a therapeutic approach for stabilizing LDLR to lower LDL-C.

VERVE-101 is currently being evaluated in the first-in-human, open-label, single-ascending-dose, phase 1b heart-1 trial in patients with heterozygous familial hypercholesterolemia (FH) and high risk for cardiovascular events. The preliminary data from the heart-1 trial were recently presented at the American Heart Association Scientific Sessions held in Philadelphia.¹⁰ A total of 10 participants were treated across 4 dose cohorts (0.1, 0.3, 0.45, and 0.6 mg/kg). Following a single intravenous infusion of VERVE-101, a dose-dependent reduction of plasma PCSK9 and LDL-C was observed in the participants. Two patients treated with 0.45 mg/kg VERVE-101 had 59% and 84% reductions of plasma PCSK9 with concomitant reduction in blood LDL-C by 39% and 48%, respectively, comparable to those treated with newly approved PCSK9 inhibitors.¹¹ The single patient treated with 0.6 mg/kg VERVE-101 showed a 47% reduction in circulating PCSK9 and a 55% reduction in blood LDL-C with durability extending to 6 months. In non-human primates, blood LDL-C was observed to be durably lowered for at least 2.5 years following a single infusion of VERVE-101 at 1.5 mg/kg.

VERVE-101 was well tolerated in the lower-dose cohorts, but treatment-related adverse events were observed in the two higher-dose cohorts. After the VERVE-101 infusion, two patients with pre-existing severely blocked arteries experienced serious heart issues. One succumbed to cardiac arrest; however, this event may not be related to the treatment, according to Verve. The other patient survived a heart attack occurring just a day after the infusion, indicating a potential association with the treatment. Notably, this individual had undisclosed chest pains before the trial.

Previous work in cultured cells and animal models showed that base editing can lead to off-target editing, raising further safety concerns. Verve presented data showing no evidence for off-target editing in primary human donor liver cells treated with a saturating dose of VERVE-101, likely attributable to the short duration of action from the LNPs-delivered base editor mRNA. However, further research is imperative to ensure the long-term safety of in vivo base-editing therapy.

Verve plans to enroll an expansion cohort including a total of about 40 patients with FH in 2024. Current participants are enrolled in New Zealand and the UK. After Verve presented data indicating that VERVE-101 would not alter DNA in reproductive cells, the FDA cleared the path for testing. Verve plans to initiate a larger randomized and placebo-controlled phase 2 trial in 2025.

The approval of CASGEVY and the promising VERVE-101 results from the heart-1 clinical trial are a beacon of hope in the field of genetic medicine. The journey toward a safer, more effective, affordable, and permanent treatment for CVD has taken a significant leap forward, marking a promising chapter in the ongoing quest for precision medicine to treat human diseases.