Aviva Bio has received formal guidance from the US Food and Drug Administration on the regulatory pathway for developing a testosterone therapy for women, alongside new scientific evidence pointing to a reduced risk of breast cancer cell proliferation linked to its lead candidate, AVA-291.

The Massachusetts-based biotechnology company said the feedback followed a Type B meeting with the FDA and set out requirements for the development of a female-specific testosterone therapy. Aviva Bio is developing AVA-291 (d3-T), a next-generation testosterone designed to retain androgen activity while resisting aromatization, a process linked to increased breast cancer risk when testosterone is converted to oestrogen in breast tissue.

The company said the absence of an FDA-approved testosterone therapy designed specifically for women has left a significant unmet medical need, with safety concerns limiting wider clinical use. The FDA acknowledged the potential breast cancer risk associated with testosterone use in women, a factor that has historically constrained development in this area.

Aviva Bio said its approach reflects a shared focus with regulators on addressing these risks through molecular design and evidence-based development strategies.

Alongside the regulatory update, the company announced that new data on AVA-291 (d3-T) have been accepted for presentation at the American Association for Cancer Research Annual Meeting in April 2026. The data show that AVA-291 has approximately 1,000-fold less potential to stimulate breast cancer cell proliferation compared with ordinary testosterone, supporting its differentiated safety profile.

“For decades, testosterone therapy for women has depended on repurposed male formulations, rather than drugs designed with women’s biology and safety considerations in mind,” said Judith A. Boice, PhD, CEO of Aviva Bio. “Our FDA feedback reinforces the need for a new standard—one anchored in safety, enabled by molecular precision, and developed with regulatory clarity from the start.”

Aviva Bio is advancing AVA-291 as part of a broader development programme covering multiple indications in both women’s and men’s health, where androgen signalling plays a clinically meaningful role but is limited by aromatization.

“The data we are presenting at AACR provides mechanistic evidence that AVA-291 (d3-T) has a reduced risk of exacerbating breast cancer compared with ordinary T,” said Bradford C. Sippy, CTO of Aviva Bio. “This builds on our prior work showing that AVA-291 is resistant to aromatization and highlights the potential clinically differentiated profile.”

The company said it is now focused on translating FDA guidance and emerging data into a structured development programme, while continuing to progress its pipeline and assess strategic collaborations to support future clinical, regulatory and commercial stages.

About Aviva Bio

Aviva Bio is a clinical-stage biotechnology company unlocking the full potential of hormone-based medicine. Through novel drug design and a focus on unmet needs, Aviva Bio is developing next-generation therapeutics to improve lives.

About AVA-291 (d3-Testosterone)

AVA-291 or d3-Testosterone (d3-T) is a novel, deuterium-substituted isotopologue of testosterone designed to resist aromatization. The aromatization of testosterone is linked with potential safety (breast cancer) and tolerability (gynecomastia) concerns associated with T therapy. In vitro studies of AVA-291 (d3-T) have demonstrated a similar metabolic profile and similar androgen receptor affinity as testosterone, but, unlike testosterone, AVA-291 is highly resistant to aromatization. In human clinical studies, d3-T was shown to be a direct substitute for ordinary testosterone.

AVA-291 (d3-T) may be a useful alternative to testosterone in clinical situations where the aromatization of testosterone limits its therapeutic potential, such as hormone replacement therapy in menopausal women, men on testosterone therapy who develop gynecomastia, addressing muscle loss and/or low-libido in patients on GLP-1 therapy, and in the treatment of ER+ breast cancer.

As a structurally identical form of testosterone, AVA-291 shares similar physical properties as T, and can be substituted in any current T formulation. AVA-291 is covered by multiple issued US patents that extend to at least 2041.