Background

This nucleoside analog reverse transcriptase inhibitor was originally developed for the treatment of human immunodeficiency virus infection. It is being repurposed based on recent work implicating the activation of retrotransposons in the pathology of amyotrophic lateral sclerosis, Alzheimer’s, and other neurodegenerative diseases. It is taken as a once-daily, oral capsule formulation. The company says TPN-101 enters the brain.

Retrotransposable elements are virus-derived pieces of DNA that make up a large part of the human genome. They are normally silenced by chromatin packing and other mechanisms. However, their tight regulation loosens with age, and there is evidence from fly and mouse models that reactivation of these elements by toxic tau species can drive neurodegeneration (Sun et al., 2018; Guo et al., 2018; Ramirez et al., 2022; Jan 2023 news).

In ALS and frontotemporal dementia, the loss of functional nuclear TDP-43 protein is thought to cause reactivation of long interspersed nuclear elements 1 (LINE1) and other retrotransposons, with expression of reverse transcriptase (e.g. Li et al., 2012; Liu et al., 2019). The subsequent processing and accumulation of RNA products triggers an antiviral immune response in cells, and neuroinflammation subsequently leads to cell death (e.g. Rodriguez et al., 2021; LaRocca et al., 2019). By inhibiting LINE1 reverse transcriptase, TPN-101 is expected to prevent this immune response and protect cells.

TPN-101 is a derivative of stavudine/3TC, an HIV treatment marketed from 1994 to 2020. The companies Oncolys BioPharma and Bristol Myers Squibb began to develop TPN-101 for HIV, but stopped in 2014 for business reasons (pipeline). In 2020, Transposon licensed the drug for neurodegeneration and other diseases.

No preclinical work is published on TPN-101, but stavudine was reported to suppress neurodegeneration, measured in part via lifespan and dendritic collapse, in fruit fly models of TDP-43 toxicity or frontotemporal dementia (Krug et al, 2017; Fort-Aznar et al., 2020).

Stavudine was reported to inhibit the NLRP3 inflammasome in blood cells from Alzheimer’s patients, and to stimulate Aβ autophagy by macrophages in vitro (La Rosa et al., 2022; La Rosa et al., 2019). An ongoing Phase 1/2 pilot study is testing stavudine’s CNS penetration and target engagement in 12 people with Alzheimer’s disease.

Findings

In HIV trials, single doses of 10-900 mg of TPN-101 were said to be well-tolerated in healthy adults (Urata et al., 2014). In a trial of 200 people with HIV, repeated dosing at 100, 200, 300, or 600 mg daily for 10 days produced mild adverse events that were nonspecific and deemed unrelated to the drug. No deaths or discontinuations were reported, and TPN-101 substantially reduced blood levels of HIV (Cotte et al., 2013).

In October 2021, a Phase 2a trial began recruiting 40 patients with C9ORF72 ALS or FTD. Participants are taking 100, 200 or 400 mg TPN-101 daily or placebo for six months, followed by six months of 400 mg open-label. The primary endpoints are safety and tolerability, with secondary measures comprising pharmacokinetics in plasma and CSF, changes in CSF and blood neurofilament light chain, and change in the ALS Functional Rating Scale-revised. The trial will also look for evidence of LINE1 transcription and biomarkers of inflammation. Running at 20 locations in the United States and Europe, the trial is fully enrolled with 42 patients, and was to run until September 2023. A detailed webinar about this trial, recorded on May 6, 2022, is publicly available on the website of NEALS, the Northeast Amyotrophic Lateral Sclerosis Consortium. Interim results from this study were announced on February 13, 2024 (press release). The company claimed effects on multiple biomarkers and on the clinical endpoint of vital capacity, after six months treatment. No data were shown.

Also in October 2021, a separate trial began in 40 people with the tauopathy progressive supranuclear palsy. This study tested 100, 200, or 400 mg/day against placebo for six months, and also offered a six-month open-label extension. Endpoints were as for the ALS/FTD trial, except that the clinical endpoint was score on the PSP rating scale. The study, at 14 sites in the U.S., concluded in December 2023. In November 2023, the company announced interim results (press release). TPN-101 at 400 mg caused an 18.4 percent reduction in CSF NfL, and a 51.6 percent reduction the inflammatory biomarker IL-6, compared to placebo. Final results were announced in a February 13, 2024, press release, and later presented in a poster at the March 2024 AD/PD conference. In the open-label extension, participants who switched from placebo to six months of 400 mg TPN-101 showed reductions in CSF NfL similar to the first treated group. After one year, NfL levels had returned to baseline, whereas IL-6 continued to decline. Worsening on the PSP rating scale slowed during the open-label period in patients who had received 400 mg continuously, but the result was not statistically significant in this small study. Of 42 patients who began the trial, 39 completed the first phase, 37 completed the open-label extension. Rates of adverse events were similar between treated and placebo groups. One serious adverse event of loss of consciousness occurred in the 100 mg TPN-101 group.

In March 2023, the company began a Phase 2 trial for children and adults with Aicardi-Goutières Syndrome, a rare inflammatory disorder that affects the brain and skin. AGS is caused by mutations in DNA repair genes. The accumulation of unrepaired DNA damage is thought to activate innate immune responses that cause inflammation and neurodegeneration. The study plans to enroll 16 patients to assess safety and changes in innate immune signaling.

For details on TPN-101 trials, see clinicaltrials.gov.