Abstract
Importance
Lipoprotein(a) (Lp[a]) is an important risk factor for atherothrombotic cardiovascular disease and aortic stenosis, for which there are no treatments approved by regulatory authorities.
Objectives
To assess adverse events and tolerability of a short interfering RNA (siRNA) designed to reduce hepatic production of apolipoprotein(a) and to assess associated changes in plasma concentrations of Lp(a) at different doses.
Design, Setting, and Participants
A single ascending dose study of SLN360, an siRNA targeting apolipoprotein(a) synthesis conducted at 5 clinical research unit sites located in the US, United Kingdom, and Australia. The study enrolled adults with Lp(a) plasma concentrations of 150 nmol/L or greater at screening and no known clinically overt cardiovascular disease. Participants were enrolled between November 18, 2020, and July 21, 2021, with last follow-up on December 29, 2021.
Interventions
Participants were randomized to receive placebo (n = 8) or single doses of SLN360 at 30 mg (n = 6), 100 mg (n = 6), 300 mg (n = 6), or 600 mg (n = 6), administered subcutaneously.
Main Outcomes and Measures
The primary outcome was evaluation of safety and tolerability. Secondary outcomes included change in plasma concentrations of Lp(a) to a maximum follow-up of 150 days.
Results
Among 32 participants who were randomized and received the study intervention (mean age, 50 [SD, 13.5] years; 17 women [53%]), 32 (100%) completed the trial. One participant experienced 2 serious adverse event episodes: admission to the hospital for headache following SARS-CoV-2 vaccination and later for complications of cholecystitis, both of which were judged to be unrelated to study drug. Median baseline Lp(a) concentrations were as follows: placebo, 238 (IQR, 203-308) nmol/L; 30-mg SLN360, 171 (IQR, 142-219) nmol/L; 100-mg SLN360, 217 (IQR, 202-274) nmol/L; 300-mg SLN360, 285 (IQR, 195-338) nmol/L; and 600-mg SLN360, 231 (IQR, 179-276) nmol/L. Maximal median changes in Lp(a) were −20 (IQR, −61 to 3) nmol/L, −89 (IQR, −119 to −61) nmol/L, −185 (IQR, −226 to −163) nmol/L, −268 (IQR, −292 to −189) nmol/L, and −227 (IQR, −270 to −174) nmol/L, with maximal median percentage changes of −10% (IQR, −16% to 1%), −46% (IQR, −64% to −40%), −86% (IQR, −92% to −82%), −96% (IQR, −98% to −89%), and −98% (IQR, −98% to −97%), for the placebo group and the 30-mg, 100-mg, 300-mg, and 600-mg SLN360 groups, respectively. The duration of Lp(a) lowering was dose dependent, persisting for at least 150 days after administration.
Conclusions and Relevance
In this phase 1 study of 32 participants with elevated Lp(a) levels and no known cardiovascular disease, the siRNA SLN360 was well tolerated, and a dose-dependent lowering of plasma Lp(a) concentrations was observed. The findings support further study to determine the safety and efficacy of this siRNA.
Trial Registration
ClinicalTrials.gov Identifier: NCT04606602; EudraCT Identifier: 2020-002471-35
Key Points
Question
Is a single subcutaneous injection of a short interfering RNA (siRNA) targeting hepatic production of apolipoprotein(a) well tolerated and associated with changes in plasma concentrations of lipoprotein(a) (Lp[a]) at different doses?
Findings
This phase 1 study of the siRNA SLN360 enrolled 32 participants with elevated Lp(a) levels. One participant experienced 2 serious adverse event episodes judged to be unrelated to study drug. The maximal median percentage change from baseline in plasma Lp(a) level over 150 days was −10%, −46%, −86%, −96%, and −98% for the placebo group and the 30-mg, 100-mg, 300-mg, and 600-mg SLN360 groups, respectively.
Meaning
In this phase 1 study of 32 participants with elevated Lp(a) levels and no known cardiovascular disease, the siRNA SLN360 was well tolerated and a dose-dependent lowering of plasma Lp(a) concentrations was observed; the findings support further study to determine the safety and efficacy of this siRNA.
This phase 1 study evaluated the safety and tolerability of 4 different doses of a short interfering RNA designed to reduce hepatic production of apolipoprotein(a) and assessed associated changes in plasma concentrations of lipoprotein(a), compared with placebo, among individuals with elevated lipoprotein(a) levels and no known cardiovascular disease.
Introduction
Lipoprotein(a) (Lp[a]) has long been recognized as a genetically determined, independent risk factor for atherothrombotic cardiovascular disease.1 Mendelian randomization studies consistently have demonstrated a causal relationship between plasma concentrations of Lp(a) and the risk of myocardial infarction, stroke, peripheral arterial disease, and cardiovascular death.2 Patients with Lp(a) concentrations in the highest decile (approximately 350 nmol/L) have a lifetime risk of cardiovascular disease equivalent to those with familial hypercholesterolemia.3,4 Elevated levels of Lp(a) also have been associated with aortic valve calcification, development and more rapid progression of aortic stenosis, and a higher rate of aortic valve replacement.5,6 Although some therapies have moderate Lp(a)-lowering effects, such as administration of niacin or proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, currently no drug treatments for elevated Lp(a) concentrations have received regulatory approval.
The LPA gene encodes for apolipoprotein(a) (apo[a]), a dominant and a rate-limiting component in the hepatic synthesis of the Lp(a) particle.7 Efforts have been underway for several years to develop RNA-targeted therapies that reduce hepatic production of apo(a) and thereby reduce plasma concentrations of Lp(a). Previous phase 1 and 2 studies have demonstrated that an antisense oligonucleotide, pelacarsen, can reduce Lp(a) plasma concentrations by up to 80% with weekly administration.8 The current study examined an alternative strategy to lowering Lp(a) using SLN360, a short interfering RNA (siRNA) to target LPA messenger RNA. SLN360 is a 19-mer siRNA covalently linked to a tri-antennary N-acetyl-galactosamine (GalNAc) moiety. The GalNAc conjugate binds to asialoglycoprotein receptors, which are expressed almost exclusively on hepatocytes. This approach results in selective uptake and concentration of SLN360 in hepatocytes, enabling the drug to bind and degrade the messenger RNA that encodes for apo(a). The current phase 1 APOLLO trial examined the tolerability of SLN360 following a single dose and measured Lp(a) concentrations to a maximum of 150 days following administration.
Methods
Study Organization and Oversight
The trial protocol was written by the sponsor (Silence Therapeutics, London, England) in collaboration with the Cleveland Clinic Coordinating Center for Clinical Research (C5Research). The trial was coordinated by the sponsor, C5Research, and a contract research organization (Medpace, Cincinnati, Ohio). The protocol and statistical analysis plan are available in Supplement 1 and Supplement 2. The study design was approved by responsible regulatory agencies and ethics committees or institutional review boards at each site prior to commencing participant enrollment. All potential participants provided written informed consent prior to study entry. C5Research, the sponsor, and Medpace provided operational management of sites and Medpace collected the data. An independent safety review committee monitored the trial and reviewed individual participant data with support from an independent data analysis center at Medpace. After database lock, Medpace transferred a copy of the database to C5Research for statistical analysis.
Study Population
Adults aged 18 years or older with no known atherosclerotic cardiovascular disease, an Lp(a) concentration of 150 nmol/L or greater (approximately >60 mg/dL), and a body mass index of 18 to 45 were eligible to participate. Individuals of childbearing potential were required to have a negative serum pregnancy test result at screening and a negative urine pregnancy test result 1 day prior to treatment. All participants (male and female) agreed to adhere to contraception requirements from the beginning of the screening period until 3 months after the last administration of study drug. Participants were excluded for moderate or severe hepatic cirrhosis, a positive test result for HIV or hepatitis B or C virus, or other liver disease that could increase the risk of drug-induced liver injury or influence the pharmacology of SLN360. Participants were required to have a hemoglobin A1c of less than 6.5% if not diagnosed previously as having diabetes or less than 8.5% if known to have diabetes.
Laboratory exclusion criteria included a platelet count below the lower limit of normal or an alanine aminotransferase (ALT) or aspartate aminotransferase (AST) more than 1.5 times the upper limit of normal. Additional laboratory exclusion criteria were total bilirubin greater than the upper limit of normal (except in participants with Gilbert syndrome) or an estimated glomerular filtration rate of less than 60 mL/min/1.73 m2. Medications that could influence Lp(a) levels required doses to be stable for at least 8 weeks prior to enrollment, including antiestrogen or estrogen receptor modulators, PCSK9 inhibitors, prescription-dose niacin, fibrates, statins, or ezetimibe. Participants were also excluded if they had received any other investigational agent within 90 days (or 10 half-lives, whichever was longer) or other oligonucleotide therapy within 12 months before the first dose of study drug. Participants were excluded for clinically significant illness within 7 days before the first dose of study drug, a positive nucleic acid test result for SARS-CoV-2, or any uncontrolled or serious disease that could interfere with the interpretation of results. Race was determined by investigators in consultation with participants using fixed categories of American Indian or Alaska Native, Asian, Black or African American, Native Hawaiian or other Pacific Islander, White, or other. These data were collected because Lp(a) concentrations are known to differ between ancestral groups.
Study Procedures
This study was conducted in accordance with the Declaration of Helsinki and Council for International Organizations of Medical Sciences International Ethical Guidelines.9 Four ascending single-dose cohorts were randomized using an interactive response technology to 30 mg, 100 mg, 300 mg, or 600 mg of SLN360 or placebo (sodium chloride, 0.9%) given by subcutaneous injection in the abdomen. Up to 2 injection sites were used to administer the required dose, with a maximum 1.5-mL injection volume administered at each site. For each cohort, 2 participants were randomly assigned to receive placebo and 6 participants to SLN360, with sentinel dosing used for the first 2 participants in each cohort. A schematic describing the study design is provided in eFigure 1 in Supplement 3. Dose escalation decisions were made on the recommendation of the safety review committee following review of safety, pharmacokinetics, and Lp(a) concentrations following drug administration. The investigators, study participants, and the sponsor were blinded to study drug assignment, but delegated site staff (eg, site pharmacist) were unblinded. Participants were monitored in a clinical research unit for 24 hours following dose administration, during which vital signs, an abbreviated physical examination, blood and urine samples, and serial electrocardiograms were obtained. Clinical research unit staff also assessed participants for treatment-emergent and injection site adverse events.
Participants had scheduled visits at days 7, 14, 30, 45, 60, 90, and 150 following drug administration. The presence and extent of any adverse events were assessed at each visit. At all return visits, blood samples were obtained for Lp(a) levels, other biomarkers, pharmacokinetic analyses, and safety laboratory studies, and, if indicated by symptoms, a focused physical examination was performed. Lipoprotein(a) concentration was measured with a particle-enhanced turbidimetric assay on a Roche c502 autoanalyzer. This assay is calibrated by 5 independent standards with values in nanomoles per liter traceable to the World Health Organization/International Federation of Clinical Chemistry and Laboratory Medicine Reference Material for Lp(a) and is minimally affected by the particle size variation of apo(a).
Study End Points
Because this is a first-in-human study, the safety and tolerability of SLN360 were defined as a primary outcome of interest. Safety assessments included changes in vital signs, physical examination, electrocardiography, and laboratory assessments of chemistry, hematology, urinalysis, and coagulation parameters. Assessments included treatment-emergent adverse events and adverse events of special interest, which included injection site adverse events or any identified dose-limiting toxicity. Injection site adverse events were graded by study personnel using both the Common Terminology Criteria for Adverse Events (CTCAE)10 version 5 and a US Food and Drug Administration (FDA)–endorsed scale11 (eTable 1 in Supplement 3).
A prespecified secondary outcome and the primary efficacy measure was plasma Lp(a) concentrations from baseline to 150 days following dosing. Other secondary outcomes included assessment of changes in lipid parameters (low-density lipoprotein, high-density lipoprotein, and total cholesterol and triglycerides) and plasma pharmacokinetics of SLN360 following administration (maximum concentration, time to maximum concentration, and area under the curve to last measurable concentration). Exploratory end points included assessment of changes in levels of apolipoprotein B, oxidized low-density lipoprotein, and 2 closely related inflammatory biomarkers, interleukin 6 and C-reactive protein. A post hoc analysis examined Lp(a) concentration changes at 150 days following drug administration.
Sample Size
There was no formal sample size calculation for this first-in-human study. For each cohort, a sample size of 8 (6 active and 2 placebo), including a sentinel pair, was selected as a feasible number of participants to study the safety and tolerability of SLN360.
Statistical Analysis
Participants randomized to placebo from each cohort were pooled to create an overall placebo treatment group. SLN360 treatment groups are presented separately by dose. Descriptive statistics are used to summarize safety and Lp(a) change by treatment group. For categorical variables, summary tabulations of frequency and percentage of participants within each category are presented. For continuous variables, the number of participants and summary statistics are presented by treatment group. Missing data were not imputed. No formal statistical testing was conducted and therefore no P values are reported. The protocol prespecified that the Lp(a) concentrations after single ascending doses of SLN360 would be reported separately from the subsequent multiple-dose portion of the trial. This report describes the findings of the single ascending dose portion. Analyses were performed using SAS version 9.4 (SAS Institute Inc).
Results
Figure 1 shows the flow of participants through the trial. Table 1 reports the baseline characteristics of the trial participants. The mean age of participants was 50 (SD, 13.5) years and 47% were male. Although none of the participants had known preexisting clinically overt cardiovascular disease, hypertension was present in 9% and diabetes in 3%. Baseline characteristics appeared broadly similar across all treatment groups. Median baseline Lp(a) concentrations were 238 (IQR, 203-308) nmol/L, 171 (IQR, 142-219) nmol/L, 217 (IQR, 202-274) nmol/L, 285 (IQR, 195-338) nmol/L, and 231 (IQR, 179-276) nmol/L for the placebo group and the 30-mg, 100-mg, 300-mg, and 600-mg SLN360 treatment groups, respectively. Mean levels of low-density lipoprotein cholesterol were 99 (SD, 48) mg/dL, 113 (SD, 38) mg/dL, 121 (SD, 46) mg/dL, 100 (SD, 25) mg/dL, and 108 (SD, 54) mg/dL for the placebo group and the 30-mg, 100-mg, 300-mg, and 600-mg SLN360 treatment groups, respectively.
Figure 1. Flow of Participants Through the APOLLO Trial.
aNo further details were recorded in case report forms. The protocol specified exclusion for many conditions, including (but not limited to) any history of clinically overt cardiovascular disease, any uncontrolled or serious disease, moderate or severe hepatic cirrhosis, active serious mental illness, and positive nucleic acid test result for SARS-CoV-2.
bWithdrawal prior to drug administration was a physician decision due to a clinically significant electrocardiogram finding recorded as an adverse event of sinus tachycardia.
Table 1. Baseline Characteristics of Participants.
| Characteristics | 30-mg SLN360 (n = 6) | 100-mg SLN360 (n = 6) | 300-mg SLN360 (n = 6) | 600-mg SLN360 (n = 6) | Placebo (n = 8) |
|---|---|---|---|---|---|
| Age, mean (SD), y | 45.5 (10.5) | 46.3 (12.3) | 58.7 (13.2) | 43.7 (17.5) | 52.9 (12.0) |
| Sex, No. (%) | |||||
| Female | 2 (33) | 2 (33) | 4 (67) | 3 (50) | 6 (75) |
| Male | 4 (67) | 4 (67) | 2 (33) | 3 (50) | 2 (25) |
| Individuals of childbearing potential, No. (%) | 0 | 0 | 0 | 0 | 1 (13) |
| Body mass index, mean (SD)a | 26 (3.3) | 29 (3.7) | 29 (4.0) | 27 (3.8) | 25 (4.0) |
| Race, No. (%)b | |||||
| Asian | 0 | 0 | 0 | 0 | 1 (13) |
| Black or African American | 5 (83) | 1 (17) | 3 (50) | 1 (17) | 1 (13) |
| White | 1 (17) | 5 (83) | 3 (50) | 5 (83) | 6 (75) |
| Hypertension, No. (%)c | 0 | 0 | 2 (33) | 1 (17) | 0 |
| Diabetes, No. (%)c | 0 | 0 | 0 | 0 | 1 (13) |
| Concomitant statin use, No. (%) | 0 | 2 (33) | 3 (50) | 3 (50) | 5 (63) |
| Lipoprotein(a), median (IQR), nmol/Ld | 171 (142-219) | 217 (202-274) | 285 (195-338) | 231 (179-276) | 238 (203-308) |
| Total cholesterol, mean (SD), mg/dLe | 197 (47) | 195 (47) | 170 (19) | 180 (51) | 185 (54) |
| Low-density lipoprotein cholesterol, mean (SD), mg/dLe | 113 (38) | 121 (46) | 100 (25) | 108 (54) | 99 (48) |
| High-density lipoprotein cholesterol, mean (SD), mg/dLe | 70 (14) | 55 (14) | 46 (9) | 55 (9) | 67 (20) |
| Triglycerides, median (IQR), mg/dLe | 55 (34-71) | 80 (63-130) | 85 (67-179) | 75 (42-115) | 93 (59-122) |
| Apolipoprotein B, mean (SD), mg/dLe | 83 (23) | 94 (29) | 89 (6) | 81 (25) | 81 (30) |
| Oxidized low-density lipoprotein, mean (SD), U/Le | 56 (18) | 64 (24) | 55 (7) | 50 (15) | 53 (22) |
SI conversion factors: To convert total, low-density lipoprotein, and high-density lipoprotein cholesterol to millimoles per liter, multiply by 0.0259; to convert triglycerides to millimoles per liter, multiply by 0.0113.
Calculated as weight in kilograms divided by height in meters squared.
Race was determined by investigators in consultation with participants. There were no American Indian, Alaska Native, Native Hawaiian, or other Pacific Islander participants.
Based on patient report and medical record review.
Normal values are less than 75 nmol/L.
Normal values are not standardized and interpretations vary. Total cholesterol less than 200 mg/dL, low-density lipoprotein cholesterol less than 100 mg/dL, high-density lipoprotein cholesterol greater than 45 mg/dL, and triglycerides less than 150 mg/dL are considered desirable.
Primary Outcome: Adverse Events
Table 2 reports adverse events for each treatment group, including assessment of injection site adverse events graded by the CTCAE scale,10 and eTable 8 in Supplement 3 reports injection site adverse events using the FDA vaccine reaction scale. Treatment-emergent adverse events were generally mild, most commonly low-grade injection site events (grades 1 and 2) and headache, with none resulting in participant withdrawal. At day 45, after receiving a SARS-CoV-2 vaccine on day 38, 1 participant in the lowest-dose group (30 mg) was admitted to the hospital for fever and a severe headache, which were ascribed by the investigator to effects of the SARS-CoV-2 vaccine. The subsequent planned day 45 visit was conducted on day 49 and the participant was found to have values for ALT and AST that were greater than 3 times the upper limit of normal, with no elevation in bilirubin. The AST and ALT values had returned to normal at the routine day 60 laboratory testing.
Table 2. Assessment of Adverse Events.
| No. (%) | |||||
|---|---|---|---|---|---|
| 30-mg SLN360 (n = 6) | 100-mg SLN360 (n = 6) | 300-mg SLN360 (n = 6) | 600-mg SLN360 (n = 6) | Placebo (n = 8) | |
| No. of participants with any treatment-emergent adverse event | 6 (100) | 6 (100) | 6 (100) | 6 (100) | 6 (75) |
| Treatment-emergent adverse events occurring in ≥3 participants | |||||
| Headache | 2 (33) | 1 (17) | 0 | 5 (83) | 1 (13) |
| Diarrhea | 1 (17) | 0 | 0 | 1 (17) | 1 (13) |
| Arthralgia | 1 (17) | 0 | 1 (17) | 1 (17) | 0 |
| Neutrophil count increased | 0 | 0 | 0 | 3 (50) | 0 |
| C-reactive protein increased | 0 | 0 | 0 | 4 (67) | 0 |
| Injection site adverse eventsa | 5 (83) | 6 (100) | 5 (83) | 6 (100) | 1 (13) |
| Grade 1 | 5 (83) | 6 (100) | 4 (67) | 2 (33) | 1 (13) |
| Grade 2 | 0 | 0 | 1 (17) | 4 (67) | 0 |
| Grade 3 | 0 | 0 | 0 | 0 | 0 |
| Serious adverse eventsb | 1 (17)b | 0 | 0 | 0 | 0 |
| Liver function | |||||
| Alanine aminotransferase >3× ULN | 1 (17)c | 0 | 0 | 0 | 0 |
| Aspartate aminotransferase >3× ULN | 1 (17)c | 0 | 0 | 0 | 0 |
| Alkaline phosphatase >2× ULN | 0 | 0 | 0 | 0 | 0 |
| Total bilirubin >2× ULN | 0 | 0 | 0 | 0 | 0 |
| QTcF prolongationd | 0 | 0 | 0 | 0 | 0 |
Abbreviation: ULN, upper limit of normal.
Graded using the Common Terminology Criteria for Adverse Events. Grade 1 is asymptomatic or mild symptoms; grade 2, moderate, with minimal, local, or noninvasive intervention indicated; and grade 3, severe or medically significant (eg, ulceration or necrosis) but not immediately life-threatening.
Serious adverse events were defined as those requiring hospitalization. One participant experienced 2 serious adverse event episodes: admission for headache following SARS-CoV-2 vaccination and, later, complications of cholecystitis, both judged unrelated to study drug.
Elevations in alanine aminotransferase and aspartate aminotransferase to >3× ULN occurred in the same participant at a single time point.
A confirmed corrected QT interval for heart rate using the Fridericia formula (QTcF) greater than 500 milliseconds and/or an increase of greater than 60 milliseconds from baseline on repeat electrocardiograms performed at least 2 minutes apart.
Secondary Outcomes
Figure 2 shows the absolute and percentage changes over time in Lp(a) concentration for each treatment group for 150 days following injection. eFigures 2A and 2B in Supplement 3 show these outcomes with individual data points for each participant. eTable 2 in Supplement 3 shows the median values and IQRs for absolute and percentage changes at each time point. The maximal median changes in Lp(a) were −20 (IQR, −61 to 3) nmol/L, −89 (IQR, −119 to −61) nmol/L, −185 (IQR, −226 to −163) nmol/L, −268 (IQR, −292 to −189) nmol/L, and −227 (IQR, −270 to −174) nmol/L for the placebo group and the 30-mg, 100-mg, 300-mg, and 600-mg SLN360 doses, respectively. The maximal median percentage changes were −10% (IQR, −16% to 1%), −46% (IQR, −64% to −40%), −86% (IQR, −92% to −82%), −96% (IQR, −98% to −89%), and −98% (IQR, −98% to −97%) for the placebo group and the 30-mg, 100-mg, 300-mg, and 600-mg SLN360 groups, respectively. The nadir of percentage change in Lp(a) levels occurred between 30 and 60 days after administration for all treatment groups (Figure 2). Concentrations gradually increased from the nadir but had not returned to baseline values by 150 days for all treatment groups. A post hoc analysis showed that median Lp(a) concentrations were more than 70% and more than 80% below baseline at the day 150 visit following administration of the 300-mg and 600-mg SLN360 doses, respectively.
Figure 2. Primary Efficacy Outcome: Changes Over Time in Lipoprotein(a) Concentration.
A, Lipoprotein(a) concentrations over time in the placebo group and each SLN360 dose group. The median lipoprotein(a) concentrations are shown as solid circles; the whiskers are IQRs for each of these values. The triangles are the mean concentrations at each time point. B, Percentage changes in lipoprotein(a) concentration over time for the placebo group and each SLN360 dose group. The median percentage changes in lipoprotein(a) concentration are shown as solid circles; the whiskers are IQRs for each of these values. The triangles are the mean percentage changes in concentration at each time point. In both panels, the median and mean values represent measurements in 6 patients for each treatment group except for the placebo group, which represents 8 participants. eFigures 2A and 2B in Supplement 3 show plots with individual data points.
SLN360 produced a dose-dependent reduction in total cholesterol and low-density lipoprotein cholesterol. Mean levels were reduced by a maximum of 18% for total cholesterol and 26% for low-density lipoprotein cholesterol, both following administration of the 600-mg SLN360 dose (Figure 3A; eTables 3 and 4 in Supplement 3). Lesser reductions were observed at lower doses. SLN360 did not reduce triglycerides or high-density lipoprotein cholesterol. eTable 5 in Supplement 3 shows plasma pharmacokinetics of SLN360.
Figure 3. Secondary and Exploratory Outcomes: Changes in Low-Density Lipoprotein Cholesterol and Apolipoprotein B.
A, Mean percentage changes in the secondary outcome, low-density lipoprotein cholesterol, over time in the placebo group and each SLN360 dose group. The mean percentage changes are shown as solid circles with the 95% CIs shown as whiskers. Baseline mean values for low-density lipoprotein cholesterol by dose group: placebo, 99 (SD, 48) mg/dL; 30-mg SLN360, 113 (SD, 38) mg/dL; 100-mg SLN360, 121 (SD, 46) mg/dL; 300-mg SLN360, 100 (SD, 25) mg/dL; 600-mg SLN360, 108 (SD, 54) mg/dL. B, Mean percentage changes in the exploratory outcome, apolipoprotein B, over time in the placebo group and each SLN360 dose group. The mean percentage changes are shown as solid circles with the 95% CIs shown as whiskers. Baseline mean values for apolipoprotein B by dose group: placebo, 81 (SD, 30) mg/dL; 30-mg SLN360, 83 (SD, 23) mg/dL; 100-mg SLN360, 94 (SD, 29) mg/dL; 300-mg SLN360, 89 (SD, 6) mg/dL; 600-mg SLN360, 81 (SD, 25) mg/dL. In both panels, the mean values represent measurements in 6 patients for each treatment group except for the placebo group, which represents 8 participants. Changes in total cholesterol contributed meaningfully to the main findings and are reported in eTable 3 in Supplement 3. Triglycerides and high-density lipoprotein cholesterol are not affected by changes in lipoprotein(a) and are not reported.
Exploratory Outcomes
Maximum reduction in the mean level of apolipoprotein B was 24% as measured 30 days after the 600-mg dose of SLN360 and 19% as measured 14 days after the 300-mg dose of SLN360 (Figure 3B; eTable 6 in Supplement 3). Mean levels of oxidized low-density lipoprotein were lowered by a maximum of 20% in the SLN360 600-mg dose group and 11% in the SLN360 300-mg dose group, with similar reductions sustained to 150 days in the 600-mg dose group (eTable 7 in Supplement 3).
Dose-dependent increases in C-reactive protein occurred, with the greatest increase observed in the SLN360 600-mg dose group, increasing from a median of 1.0 mg/L at baseline to 3.6 mg/L at day 7, then rapidly decreasing (eTable 9 in Supplement 3). Neutrophils also increased during the first 24 hours, returning to normal levels by the day 7 visit.
Discussion
In this phase 1 study of 32 participants with Lp(a) levels greater than 150 nmol/L and no known cardiovascular disease, SLN360, an siRNA targeting apo(a) production, was well tolerated and showed dose-dependent lowering of plasma Lp(a) concentrations.
Patients with Lp(a) concentrations greater than 150 nmol/L (approximately 60 mg/dL) comprise approximately 20% of the general population and are associated with greatly increased risk of developing atherosclerotic cardiovascular disease and aortic valve stenosis.4,5 Plasma Lp(a) levels are genetically determined and are not influenced by lifestyle interventions, such as diet and exercise, that reduce low-density lipoprotein cholesterol and triglycerides or increase high-density lipoprotein cholesterol. In the absence of an effective Lp(a)-lowering drug therapy, this risk factor has traditionally been considered essentially untreatable. Recent developments using therapeutic oligonucleotides have made it possible to selectively and substantially reduce Lp(a). The first of these drugs to reach phase 3, an antisense oligonucleotide, pelacarsen, reduced Lp(a) by up to 80% with weekly injections in a phase 2 trial with subsequent development as a monthly therapy.8
The current trial examined an alternative approach using an siRNA, SLN360, that degrades the messenger RNA that encodes for production of apo(a).12 The results of the current phase 1 study show a dose-dependent reduction in Lp(a) concentration persisting up to 150 days (Table 2 and Figure 2). Following data review, the safety review committee recommended extending follow-up of Lp(a) levels for participants in the highest 2 dose groups from 150 days to 1 year. Longer exposure and repeat dosing will be required to assess the duration of Lp(a) lowering and to comprehensively evaluate drug safety. The observed reductions in low-density lipoprotein cholesterol and apolipoprotein B were expected because current assays also measure a portion of the low-density lipoprotein cholesterol and apolipoprotein B contained within Lp(a) particles.13,14
In phase 1 studies, evaluation of adverse events and tolerability is the principal objective. The current study was small, including only 32 participants without known cardiovascular disease studied for 150 days after a single administration of SLN360. During the study, 1 episode of a transient 3-fold elevation in liver enzymes (ALT and AST) was observed in a single participant who received the lowest dose (30 mg), temporally associated with SARS-CoV-2 vaccination. Elevation of liver enzymes has been previously described after SARS-CoV-2 vaccination.15 Definitive conclusions regarding hepatic adverse events are not possible since only a small number of participants were treated for a short period of time.
Similar to other RNA interference therapies, low-grade injection site adverse events were observed, which were self-limiting and did not lead to participant withdrawal (Table 2). Transient, dose-dependent increases in C-reactive protein and neutrophils were observed during the first 7 days after administration. During the initial development of some prior antisense oligonucleotide therapies, thrombocytopenia was observed at higher weekly dosing, but no clinically relevant platelet reductions were observed in the current study.16 Future studies treating participants with multiple doses for a longer exposure time will be required to determine the safety of this siRNA therapeutic agent.
RNA interference is increasingly being used in therapeutic drug development to inhibit hepatic production of specific target proteins.12 An siRNA is a double-stranded RNA with guide (antisense) and passenger (sense) strands designed to suppress the translation of a specific target gene. Once the siRNA enters the cell, it is incorporated into an RNA-induced silencing complex. The guide strand and passenger strand are separated, and the passenger strand is cleaved and degraded. The remaining siRNA guide strand binds to the target mRNA, which is degraded by cellular exonucleases, leaving the guide strand intact to degrade additional copies of the target mRNA, resulting in durable effects. A recently published phase 1 study of olpasiran, a different siRNA targeting Lp(a), also showed large, dose-dependent, and durable lowering of Lp(a).17
The siRNA used in this trial was conjugated with GalNAc, a sugar derived from galactose, enabling targeted delivery of SLN360 to the liver. This approach is now used in most hepatically directed RNA interference therapeutics currently marketed or under development. GalNAc binds to asialoglycoprotein receptors, which are highly expressed on the surface of hepatocytes, leading to rapid endocytosis of the conjugate. The selective uptake in hepatocytes reduces the administered dose of this siRNA needed to degrade LPA mRNA, which limits systemic exposure.
Limitations
The study has several limitations. First, and typical of a first-in-human study, this phase 1 study was very small, and the ability to assess the safety of SLN360 is limited. Second, this select population may not represent results for this therapy in a more diverse population. Third, the trial studied participants without known cardiovascular disease. A population with atherosclerotic cardiovascular disease may show different results.
Conclusions
In this phase 1 study of 32 participants with elevated Lp(a) levels and no known cardiovascular disease, the siRNA SLN360 was well tolerated, and a dose-dependent reduction in plasma Lp(a) levels was observed. The findings support further study to determine the safety and efficacy of this siRNA.
Trial Protocol
Statistical Analysis Plan
eFigure 1. Study Schematic
eFigure 2A. Secondary Outcome and Primary Efficacy Endpoint: Lipoprotein(a) Concentrations From Baseline to 150 Days After Administration
eFigure 2B. Secondary Outcome and Primary Efficacy Endpoint: Percent Change in Lipoprotein(a) Concentration From Baseline to 150 Days After Administration
eTable 1. FDA Vaccine Product Injection Site Reaction Grading Scale
eTable 2. Secondary Outcome and Primary Efficacy Endpoint: Change in Lipoprotein (a) Concentration Over Time
eTable 3. Changes in Total Cholesterol Following SLN360 Administration
eTable 4. Changes in Low-Density Lipoprotein Cholesterol Following SLN360 Administration
eTable 5. Pharmacokinetic Parameters
eTable 6. Changes in Apolipoprotein(B) Following SLN360 Administration
eTable 7. Changes in Oxidized LDL Following SLN360 Administration
eTable 8. Injection Site Adverse Events 24 Hours After Administration Graded Using the FDA Vaccine Product Injection Site Reaction Grading Scale
eTable 9. Changes in C-Reactive Protein After Administration of SLN360
Data Sharing Statement
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Trial Protocol
Statistical Analysis Plan
eFigure 1. Study Schematic
eFigure 2A. Secondary Outcome and Primary Efficacy Endpoint: Lipoprotein(a) Concentrations From Baseline to 150 Days After Administration
eFigure 2B. Secondary Outcome and Primary Efficacy Endpoint: Percent Change in Lipoprotein(a) Concentration From Baseline to 150 Days After Administration
eTable 1. FDA Vaccine Product Injection Site Reaction Grading Scale
eTable 2. Secondary Outcome and Primary Efficacy Endpoint: Change in Lipoprotein (a) Concentration Over Time
eTable 3. Changes in Total Cholesterol Following SLN360 Administration
eTable 4. Changes in Low-Density Lipoprotein Cholesterol Following SLN360 Administration
eTable 5. Pharmacokinetic Parameters
eTable 6. Changes in Apolipoprotein(B) Following SLN360 Administration
eTable 7. Changes in Oxidized LDL Following SLN360 Administration
eTable 8. Injection Site Adverse Events 24 Hours After Administration Graded Using the FDA Vaccine Product Injection Site Reaction Grading Scale
eTable 9. Changes in C-Reactive Protein After Administration of SLN360
Data Sharing Statement