30 Years of Biotherapeutics Development-What Have We Learned?

Since the birth of biotechnology, hundreds of biotherapeutics have been developed and approved by the US Food and Drug Administration (FDA) for human use. These novel medicines not only bring significant benefit to patients but also represent precision tools to interrogate human disease biology. Accordingly, much has been learned from the successes and failures of hundreds of high-quality clinical trials. In this review, we discuss general and broadly applicable themes that have emerged from this collective experience. We base our discussion on insights gained from exploring some of the most important target classes, including interleukin-1 (IL-1), tumor necrosis factor α (TNF-α), IL-6, IL-12/23, IL-17, IL-4/13, IL-5, immunoglobulin E (IgE), integrins and B cells. We also describe current challenges and speculate about how emerging technological capabilities may enable the discovery and development of the next generation of biotherapeutics.

Mitigation of chromosome loss in clinical CRISPR-Cas9-engineered T cells.

CRISPR-Cas9 genome editing has enabled advanced T cell therapies, but occasional loss of the targeted chromosome remains a safety concern. To investigate whether Cas9-induced chromosome loss is a universal phenomenon and evaluate its clinical significance, we conducted a systematic analysis in primary human T cells. Arrayed and pooled CRISPR screens revealed that chromosome loss was generalizable across the genome and resulted in partial and entire loss of the targeted chromosome, including in preclinical chimeric antigen receptor T cells. T cells with chromosome loss persisted for weeks in culture, implying the potential to interfere with clinical use. A modified cell manufacturing process, employed in our first-in-human clinical trial of Cas9-engineered T cells (NCT03399448), reduced chromosome loss while largely preserving genome editing efficacy. Expression of p53 correlated with protection from chromosome loss observed in this protocol, suggesting both a mechanism and strategy for T cell engineering that mitigates this genotoxicity in the clinic.

The next horizon in precision oncology: Proteogenomics to inform cancer diagnosis and treatment.

When it comes to precision oncology, proteogenomics may provide better prospects to the clinical characterization of tumors, help make a more accurate diagnosis of cancer, and improve treatment for patients with cancer. This perspective describes the significant contributions of The Cancer Genome Atlas and the Clinical Proteomic Tumor Analysis Consortium to precision oncology and makes the case that proteogenomics needs to be fully integrated into clinical trials and patient care in order for precision oncology to deliver the right cancer treatment to the right patient at the right dose and at the right time.

Maternal Fecal Microbiota Transplantation in Cesarean-Born Infants Rapidly Restores Normal Gut Microbial Development: A Proof-of-Concept Study.

Infants born by vaginal delivery are colonized with maternal fecal microbes. Cesarean section (CS) birth disturbs mother-to-neonate transmission. In this study (NCT03568734), we evaluated whether disturbed intestinal microbiota development could be restored in term CS-born infants by postnatal, orally delivered fecal microbiota transplantation (FMT). We recruited 17 mothers, of whom seven were selected after careful screening. Their infants received a diluted fecal sample from their own mothers, taken 3 weeks prior to delivery. All seven infants had an uneventful clinical course during the 3-month follow-up and showed no adverse effects. The temporal development of the fecal microbiota composition of FMT-treated CS-born infants no longer resembled that of untreated CS-born infants but showed significant similarity to that of vaginally born infants. This proof-of-concept study demonstrates that the intestinal microbiota of CS-born infants can be restored postnatally by maternal FMT. However, this should only be done after careful clinical and microbiological screening.

Mutational Landscape of Secondary Glioblastoma Guides MET-Targeted Trial in Brain Tumor.

Low-grade gliomas almost invariably progress into secondary glioblastoma (sGBM) with limited therapeutic option and poorly understood mechanism. By studying the mutational landscape of 188 sGBMs, we find significant enrichment of TP53 mutations, somatic hypermutation, MET-exon-14-skipping (METex14), PTPRZ1-MET (ZM) fusions, and MET amplification. Strikingly, METex14 frequently co-occurs with ZM fusion and is present in ∼14% of cases with significantly worse prognosis. Subsequent studies show that METex14 promotes glioma progression by prolonging MET activity. Furthermore, we describe a MET kinase inhibitor, PLB-1001, that demonstrates remarkable potency in selectively inhibiting MET-altered tumor cells in preclinical models. Importantly, this compound also shows blood-brain barrier permeability and is subsequently applied in a phase I clinical trial that enrolls MET-altered chemo-resistant glioma patients. Encouragingly, PLB-1001 achieves partial response in at least two advanced sGBM patients with rarely significant side effects, underscoring the clinical potential for precisely treating gliomas using this therapy.

LXR/ApoE Activation Restricts Innate Immune Suppression in Cancer.

Therapeutic harnessing of adaptive immunity via checkpoint inhibition has transformed the treatment of many cancers. Despite unprecedented long-term responses, most patients do not respond to these therapies. Immunotherapy non-responders often harbor high levels of circulating myeloid-derived suppressor cells (MDSCs)-an immunosuppressive innate cell population. Through genetic and pharmacological approaches, we uncovered a pathway governing MDSC abundance in multiple cancer types. Therapeutic liver-X nuclear receptor (LXR) agonism reduced MDSC abundance in murine models and in patients treated in a first-in-human dose escalation phase 1 trial. MDSC depletion was associated with activation of cytotoxic T lymphocyte (CTL) responses in mice and patients. The LXR transcriptional target ApoE mediated these effects in mice, where LXR/ApoE activation therapy elicited robust anti-tumor responses and also enhanced T cell activation during various immune-based therapies. We implicate the LXR/ApoE axis in the regulation of innate immune suppression and as a target for enhancing the efficacy of cancer immunotherapy in patients.

Pharmacogenomics of GPCR Drug Targets.

Natural genetic variation in the human genome is a cause of individual differences in responses to medications and is an underappreciated burden on public health. Although 108 G-protein-coupled receptors (GPCRs) are the targets of 475 (∼34%) Food and Drug Administration (FDA)-approved drugs and account for a global sales volume of over 180 billion US dollars annually, the prevalence of genetic variation among GPCRs targeted by drugs is unknown. By analyzing data from 68,496 individuals, we find that GPCRs targeted by drugs show genetic variation within functional regions such as drug- and effector-binding sites in the human population. We experimentally show that certain variants of µ-opioid and Cholecystokinin-A receptors could lead to altered or adverse drug response. By analyzing UK National Health Service drug prescription and sales data, we suggest that characterizing GPCR variants could increase prescription precision, improving patients' quality of life, and relieve the economic and societal burden due to variable drug responsiveness. VIDEO ABSTRACT.

Hear and Now: Ongoing Clinical Trials to Prevent Drug-Induced Hearing Loss.

Each year over half a million people experience permanent hearing loss caused by treatment with therapeutic drugs with ototoxic side effects. There is a major unmet clinical need for therapies that protect against this hearing loss without reducing the therapeutic efficacy of these lifesaving drugs. At least 17 clinical trials evaluating 10 therapeutics are currently underway for therapies aimed at preventing aminoglycoside- and/or cisplatin-induced ototoxicity. This review describes the preclinical and clinical development of each of these approaches, provides updates on the status of ongoing trials, and highlights the importance of appropriate outcome measures in trial design and the value of reporting criteria in the dissemination of results.

LP(a): Structure, Genetics, Associated Cardiovascular Risk, and Emerging Therapeutics.

Lipoprotein(a) [Lp(a)] is a molecule bound to apolipoprotein(a) with some similarity to low-density lipoprotein cholesterol (LDL-C), which has been found to be a risk factor for cardiovascular disease (CVD). Lp(a) appears to induce inflammation, atherogenesis, and thrombosis. Approximately 20% of the world's population has increased Lp(a) levels, determined predominantly by genetics. Current clinical practices for the management of dyslipidemia are ineffective in lowering Lp(a) levels. Evolving RNA-based therapeutics, such as the antisense oligonucleotide pelacarsen and small interfering RNA olpasiran, have shown promising results in reducing Lp(a) levels. Phase III pivotal cardiovascular outcome trials [Lp(a)HORIZON and OCEAN(a)] are ongoing to evaluate their efficacy in secondary prevention of major cardiovascular events in patients with elevated Lp(a). The future of cardiovascular residual risk reduction may transition to a personalized approach where further lowering of either LDL-C, triglycerides, or Lp(a) is selected after high-intensity statin therapy based on the individual risk profile and preferences of each patient.

Clinical translation of pluripotent stem cell-based therapies: successes and challenges.

The translational stem cell research field has progressed immensely in the past decade. Development and refinement of differentiation protocols now allows the generation of a range of cell types, such as pancreatic ß-cells and dopaminergic neurons, from human pluripotent stem cells (hPSCs) in an efficient and good manufacturing practice-compliant fashion. This has led to the initiation of several clinical trials using hPSC-derived cells to replace lost or dysfunctional cells, demonstrating evidence of both safety and efficacy. Here, we highlight successes from some of the hPSC-based trials reporting early signs of efficacy and discuss common challenges in clinical translation of cell therapies.

Emerging Roles and Therapeutic Applications of Arachidonic Acid Pathways in Cardiometabolic Diseases.

Cardiometabolic disease has become a major health burden worldwide, with sharply increasing prevalence but highly limited therapeutic interventions. Emerging evidence has revealed that arachidonic acid derivatives and pathway factors link metabolic disorders to cardiovascular risks and intimately participate in the progression and severity of cardiometabolic diseases. In this review, we systemically summarized and updated the biological functions of arachidonic acid pathways in cardiometabolic diseases, mainly focusing on heart failure, hypertension, atherosclerosis, nonalcoholic fatty liver disease, obesity, and diabetes. We further discussed the cellular and molecular mechanisms of arachidonic acid pathway-mediated regulation of cardiometabolic diseases and highlighted the emerging clinical advances to improve these pathological conditions by targeting arachidonic acid metabolites and pathway factors.

Phase-contrast virtual chest radiography.

Respiratory X-ray imaging enhanced by phase contrast has shown improved airway visualization in animal models. Limitations in current X-ray technology have nevertheless hindered clinical translation, leaving the potential clinical impact an open question. Here, we explore phase-contrast chest radiography in a realistic in silico framework. Specifically, we use preprocessed virtual patients to generate in silico chest radiographs by Fresnel-diffraction simulations of X-ray wave propagation. Following a reader study conducted with clinical radiologists, we predict that phase-contrast edge enhancement will have a negligible impact on improving solitary pulmonary nodule detection (6 to 20 mm). However, edge enhancement of bronchial walls visualizes small airways (< 2 mm), which are invisible in conventional radiography. Our results show that phase-contrast chest radiography could play a future role in observing small-airway obstruction (e.g., relevant for asthma or early-stage chronic obstructive pulmonary disease), which cannot be directly visualized using current clinical methods, thereby motivating the experimental development needed for clinical translation. Finally, we discuss quantitative requirements on distances and X-ray source/detector specifications for clinical implementation of phase-contrast chest radiography.

Effect of Dapagliflozin Versus Placebo on Symptoms and 6-Minute Walk Distance in Patients With Heart Failure: The DETERMINE Randomized Clinical Trials.

BACKGROUND: Sodium-glucose cotransporter 2 inhibitors reduce the risk of worsening heart failure (HF) and cardiovascular death in patients with HF irrespective of left ventricular ejection fraction. It is important to determine whether therapies for HF improve symptoms and functional capacity. METHODS: The DETERMINE (Dapagliflozin Effect on Exercise Capacity Using a 6-Minute Walk Test in Patients With Heart Failure) double-blind, placebo-controlled, multicenter trials assessed the efficacy of the sodium-glucose cotransporter 2 inhibitor dapagliflozin on the Total Symptom Score (TSS) and Physical Limitation Scale (PLS) of the Kansas City Cardiomyopathy Questionnaire (KCCQ) and 6-minute walk distance (6MWD) in 313 patients with HF with reduced ejection fraction (DETERMINE-Reduced) and in 504 patients with HF with preserved ejection fraction (DETERMINE-Preserved) with New York Heart Association class II or III symptoms and elevated natriuretic peptide levels. The primary outcomes were changes in the KCCQ-TSS, KCCQ-PLS, and 6MWD after 16 weeks of treatment. RESULTS: Among the 313 randomized patients with HF with reduced ejection fraction, the median placebo-corrected difference in KCCQ-TSS from baseline at 16 weeks was 4.2 (95% CI, 1.0, 8.2; P=0.022) in favor of dapagliflozin. The median placebo-corrected difference in KCCQ-PLS was 4.2 (95% CI, 0.0, 8.3; P=0.058). The median placebo-corrected difference in 6MWD from baseline at 16 weeks was 3.2 meters (95% CI, -6.5, 13.0; P=0.69). In the 504 patients with HF with preserved ejection fraction, the median placebo-corrected 16-week difference in KCCQ-TSS and KCCQ-PLS was 3.2 (95% CI, 0.4, 6.0; P=0.079) and 3.1 (-0.1, 5.4; P=0.23), respectively. The median 16-week difference in 6MWD was 1.6 meters (95% CI, -5.9, 9.0; P=0.67). In an exploratory post hoc analysis of both trials combined (DETERMINE-Pooled), the median placebo-corrected difference from baseline at 16 weeks was 3.7 (1.5, 5.9; P=0.005) for KCCQ-TSS, 4.0 (0.3, 4.9; P=0.036) for KCCQ-PLS, and 2.5 meters (-3.5, 8.4; P=0.50) for 6MWD. CONCLUSIONS: Dapagliflozin improved the KCCQ-TSS in patients with HF with reduced ejection fraction but did not improve KCCQ-PLS or 6MWD. Dapagliflozin did not improve these outcomes in patients with HF with preserved ejection fraction. In a post hoc analysis including all patients across the full spectrum of ejection fraction, there was a beneficial effect of dapagliflozin on KCCQ-TSS and KCCQ-PLS but not 6MWD. REGISTRATION: URL: https://www.clinicaltrials.gov; Unique identifiers: NCT03877237 and NCT03877224.

Ertugliflozin for Functional Mitral Regurgitation Associated With Heart Failure: EFFORT Trial.

BACKGROUND: The morbidity and mortality rates of patients with heart failure (HF) and functional mitral regurgitation (MR) remain substantial despite guideline-directed medical therapy for HF. We evaluated the efficacy of ertugliflozin for reduction of functional MR associated with HF with mild to moderately reduced ejection fraction. METHODS: The EFFORT trial (Ertugliflozin for Functional Mitral Regurgitation) was a multicenter, double-blind, randomized trial to examine the hypothesis that the sodium-glucose cotransporter 2 inhibitor ertugliflozin is effective for improving MR in patients with HF with New York Heart Association functional class II or III, 35%≤ejection fraction<50%, and effective regurgitant orifice area of chronic functional MR >0.1 cm2 on baseline echocardiography. We randomly assigned 128 patients to receive either ertugliflozin or placebo in addition to guideline-directed medical therapy for HF. The primary end point was change in effective regurgitant orifice area of functional MR from baseline to the 12-month follow-up. Secondary end points included changes in regurgitant volume, left ventricular (LV) volume indices, left atrial volume index, LV global longitudinal strain, and NT-proBNP (N-terminal pro-B-type natriuretic peptide). RESULTS: The treatment groups were generally well-balanced with regard to baseline characteristics: mean age, 66±11 years; 61% men; 13% diabetes; 51% atrial fibrillation; 43% use of angiotensin receptor-neprilysin inhibitor; ejection fraction, 42±8%; and effective regurgitant orifice area, 0.20±0.12 cm2. The decrease in effective regurgitant orifice area was significantly greater in the ertugliflozin group than in the placebo group (-0.05±0.06 versus 0.03±0.12 cm2; P<0.001). Compared with placebo, ertugliflozin significantly reduced regurgitant volume by 11.2 mL (95% CI, -16.1 to -6.3; P=0.009), left atrial volume index by 6.0 mL/m2 (95% CI, -12.16 to 0.15; P=0.005), and LV global longitudinal strain by 1.44% (95% CI, -2.42% to -0.46%; P=0.004). There were no significant between-group differences regarding changes in LV volume indices, ejection fraction, or NT-proBNP levels. Serious adverse events occurred in one patient (1.6%) in the ertugliflozin group and 6 (9.2%) in the placebo group (P=0.12). CONCLUSIONS: Among patients with functional MR associated with HF, ertugliflozin significantly improved LV global longitudinal strain and left atrial remodeling, and reduced functional MR. Sodium-glucose cotransporter 2 inhibitors may be considered for patients with functional MR. REGISTRATION: URL: https://www.clinicaltrials.gov; Unique identifier: NCT04231331.

Efficacy and Safety of Non-Vitamin-K Antagonist Oral Anticoagulants Versus Warfarin Across the Spectrum of Body Mass Index and Body Weight: An Individual Patient Data Meta-Analysis of 4 Randomized Clinical Trials of Patients With Atrial Fibrillation.

BACKGROUND: The efficacy and safety of non-vitamin-K antagonist oral anticoagulants (NOACs) across the spectrum of body mass index (BMI) and body weight (BW) remain uncertain. METHODS: We analyzed data from COMBINE AF (A Collaboration Between Multiple Institutions to Better Investigate Non-Vitamin K Antagonist Oral Anticoagulant Use in Atrial Fibrillation), which pooled patient-level data from the 4 pivotal randomized trials of NOAC versus warfarin in patients with atrial fibrillation. The primary efficacy and safety outcomes were stroke or systemic embolic events (stroke/SEE) and major bleeding, respectively; secondary outcomes were ischemic stroke/SEE, intracranial hemorrhage, death, and the net clinical outcome (stroke/SEE, major bleeding, or death). Each outcome was examined across BMI and BW. Because few patients had a BMI <18.5 kg/m2 (n=598), the primary analyses were restricted to those with a BMI ≥18.5 kg/m2. RESULTS: Among 58 464 patients, the median BMI was 28.3 (interquartile range, 25.2-32.2) kg/m2, and the median BW was 81.0 (interquartile range, 70.0-94.3) kg. The event probability of stroke/SEE was lower at a higher BMI irrespective of treatment, whereas the probability of major bleeding was lower at a higher BMI with warfarin but relatively unchanged across BMI with NOACs. NOACs reduced stroke/SEE overall (adjusted hazard ratio [HRadj], 0.80 [95% CI, 0.73-0.88]; P<0.001), with a generally consistent effect across BMI (Ptrend across HRs, 0.48). NOACs also reduced major bleeding overall (HRadj, 0.88 [95% CI, 0.82-0.94]; P<0.001), but with attenuation of the benefit at a higher BMI (trend test across BMI [Ptrend], 0.003). The overall treatment effects of NOACs versus warfarin for secondary outcomes were consistent across BMI, with the exception of the net clinical outcome and death. While these outcomes were overall reduced with NOACs (net clinical outcome, HRadj, 0.91 [95% CI, 0.87-0.95]; P<0.001; death, HRadj, 0.91 [95% CI, 0.86-0.97]; P=0.003), these benefits were attenuated at higher BMI (Ptrend, 0.001 and 0.08, respectively). All findings were qualitatively similar when analyzed across BW. CONCLUSIONS: The treatment effect of NOACs versus warfarin in atrial fibrillation is generally consistent for stroke/SEE across the spectrum of BMI and BW, whereas the reduction in major bleeding is attenuated in those with higher BMI or BW. Death and the net clinical outcome are overall reduced with NOACs over warfarin, although there remain uncertainties for these outcomes at a very high BMI and BW.

Effects of an Intervention to Improve Evidence-Based Care for People With Diabetes and Cardiovascular Disease Across Sex, Race, and Ethnicity Subgroups: Insights From the COORDINATE-Diabetes Trial.

BACKGROUND: Results from the COORDINATE-Diabetes trial demonstrated that a multifaceted, clinic-based intervention increased prescription of evidence-based medical therapies to participants with type 2 diabetes and atherosclerotic cardiovascular disease. This secondary analysis assessed whether intervention success was consistent across sex, race, and ethnicity. METHODS: COORDINATE-Diabetes, a cluster randomized trial, recruited participants from 43 US cardiology clinics (20 randomized to intervention and 23 randomized to usual care). The primary outcome was the proportion of participants prescribed all 3 groups of evidence-based therapy (high-intensity statin, angiotensin-converting enzyme inhibitor/angiotensin receptor blocker, and sodium-glucose cotransporter-2 inhibitor or glucagon-like peptide 1 receptor agonist) at last trial assessment (6 to 12 months). In this prespecified analysis, mixed-effects logistic regression models were used to assess the outcome by self-reported sex, race, and ethnicity in the intervention and usual care groups, with adjustment for baseline characteristics, medications, comorbidities, and site location. RESULTS: Among 1045 participants with type 2 diabetes and atherosclerotic cardiovascular disease, the median age was 70 years, 32% were female, 16% were Black, and 9% were Hispanic. At the last trial assessment, there was an absolute increase in the proportion of participants prescribed all 3 groups of evidence-based therapy in women (36% versus 15%), Black participants (41% versus 18%), and Hispanic participants (46% versus 18%) with the intervention compared with usual care, with consistent benefit across sex (male versus female; Pinteraction=0.44), race (Black versus White; Pinteraction=0.59), and ethnicity (Hispanic versus Non-Hispanic; Pinteraction= 0.78). CONCLUSIONS: The COORDINATE-Diabetes intervention successfully improved delivery of evidence-based care, regardless of sex, race, or ethnicity. Widespread dissemination of this intervention could improve equitable health care quality, particularly among women and minority communities who are frequently underrepresented in clinical trials. REGISTRATION: URL: https://clinicaltrials.gov. Unique identifier: NCT03936660.

Emerging Therapeutics, Technologies, and Drug Development Strategies to Address Patient Nonadherence and Improve Tuberculosis Treatment.

Imperfect medication adherence remains the biggest predictor of treatment failure for patients with tuberculosis. Missed doses during treatment lead to relapse, tuberculosis resistance, and further spread of disease. Understanding individual patient phenotypes, population pharmacokinetics, resistance development, drug distribution to tuberculosis lesions, and pharmacodynamics at the site of infection is necessary to fully measure the impact of adherence on patient outcomes. To decrease the impact of expected variabilityin drug intake on tuberculosis outcomes, an improvement in patient adherence and new forgiving regimens that protect against missed doses are needed. In this review, we summarize emerging technologies to improve medication adherence in clinical practice and provide suggestions on how digital adherence technologies can be incorporated in clinical trials and practice and the drug development pipeline that will lead to more forgiving regimens and benefit patients suffering from tuberculosis.

Advancing Parkinson's disease treatment: cell replacement therapy with neurons derived from pluripotent stem cells.

The motor symptoms of Parkinson's disease (PD) are caused by the progressive loss of dopamine neurons from the substantia nigra. There are currently no treatments that can slow or reverse the neurodegeneration. To restore the lost neurons, international groups have initiated clinical trials using human embryonic or induced pluripotent stem cells (PSCs) to derive dopamine neuron precursors that are used as transplants to replace the lost neurons. Proof of principle experiments in the 1980s and 1990s showed that grafts of fetal ventral mesencephalon, which contains the precursors of the substantial nigra, could, under rare circumstances, reverse symptoms of the disease. Improvements in PSC technology and genomics have inspired researchers to design clinical trials using PSC-derived dopamine neuron precursors as cell replacement therapy for PD. We focus here on four such first-in-human clinical trials that have begun in the US, Europe, and Japan. We provide an overview of the sources of PSCs and the methods used to generate cells for transplantation. We discuss pros and cons of strategies for allogeneic, immune-matched, and autologous approaches and novel methods for overcoming rejection by the immune system. We consider challenges for safety and efficacy of the cells for durable engraftment, focusing on the genomics-based quality control methods to assure that the cells will not become cancerous. Finally, since clinical trials like these have never been undertaken before, we comment on the value of cooperation among rivals to contribute to advancements that will finally provide relief for the millions suffering from the symptoms of PD.

Clinical Potential of Adrenomedullin Signaling in the Cardiovascular System.

Numerous clinical studies have revealed the utility of circulating AM (adrenomedullin) or MR-proAM (mid-regional proAM 45-92) as an effective prognostic and diagnostic biomarker for a variety of cardiovascular-related pathophysiologies. Thus, there is strong supporting evidence encouraging the exploration of the AM-CLR (calcitonin receptor-like receptor) signaling pathway as a therapeutic target. This is further bolstered because several drugs targeting the shared CGRP (calcitonin gene-related peptide)-CLR pathway are already Food and Drug Administration-approved and on the market for the treatment of migraine. In this review, we summarize the AM-CLR signaling pathway and its modulatory mechanisms and provide an overview of the current understanding of the physiological and pathological roles of AM-CLR signaling and the yet untapped potentials of AM as a biomarker or therapeutic target in cardiac and vascular diseases and provide an outlook on the recently emerged strategies that may provide further boost to the possible clinical applications of AM signaling.

Vitamin D did not reduce multiple sclerosis disease activity after a clinically isolated syndrome.

Low serum levels of 25-hydroxyvitamin D [25(OH)D] and low sunlight exposure are known risk factors for the development of multiple sclerosis. Add-on vitamin D supplementation trials in established multiple sclerosis have been inconclusive. The effects of vitamin D supplementation to prevent multiple sclerosis is unknown. We aimed to test the hypothesis that oral vitamin D3 supplementation in high-risk clinically isolated syndrome (abnormal MRI, at least three T2 brain and/or spinal cord lesions), delays time to conversion to definite multiple sclerosis, that the therapeutic effect is dose-dependent, and that all doses are safe and well tolerated. We conducted a double-blind trial in Australia and New Zealand. Eligible participants were randomized 1:1:1:1 to placebo, 1000, 5000 or 10 000 international units (IU) of oral vitamin D3 daily within each study centre (n = 23) and followed for up to 48 weeks. Between 2013 and 2021, we enrolled 204 participants. Brain MRI scans were performed at baseline, 24 and 48 weeks. The main study outcome was conversion to clinically definite multiple sclerosis based on the 2010 McDonald criteria defined as either a clinical relapse or new brain MRI T2 lesion development. We included 199 cases in the intention-to-treat analysis based on assigned dose. Of these, 116 converted to multiple sclerosis by 48 weeks (58%). Compared to placebo, the hazard ratios (95% confidence interval) for conversion were 1000 IU 0.87 (0.50, 1.50); 5000 IU 1.37 (0.82, 2.29); and 10 000 IU 1.28 (0.76, 2.14). In an adjusted model including age, sex, latitude, study centre and baseline symptom number, clinically isolated syndrome onset site, presence of infratentorial lesions and use of steroids, the hazard ratios (versus placebo) were 1000 IU 0.80 (0.45, 1.44); 5000 IU 1.36 (0.78, 2.38); and 10 000 IU 1.07 (0.62, 1.85). Vitamin D3 supplementation was safe and well tolerated. We did not demonstrate reduction in multiple sclerosis disease activity by vitamin D3 supplementation after a high-risk clinically isolated syndrome.