Ambrx tries another IPO as it chases down HER2 rivals

California biotech Ambrx Biopharma is hoping to raise $126 million from a listing on the New York Stock Exchange (NYSE) that will be used in part to fund clinical development of ARX-788, its HER2-targeting antibody drug conjugate (ADC).

The Scripps Research spinout could be just 18 months away from licensee NovoCodex reporting results of a Chinese phase 2/3 trial of ARX788 as a later-line therapy for HER2-positive breast cancer patients which – if positive – could set the drug on course for regulatory approval in China.

In the US, Ambrx has started recruiting patients into a phase 2 study involving patients previously treated with other HER2 drugs including Roche's Kadcyla (trastuzumab emtansine), AstraZeneca/Daiichi Sankyo's Enhertu (trastuzumab deruxtecan) or Seattle Genetics orally-active Tukysa (tucatinib). ARX788 has fast-track status from the FDA for that indication.

Almost $90 million of the proceeds from the IPO would be used to the US phase 2 trial as well as phase 1 studies in HER2-positive gastric cancer and other solid tumours – all indications that AZ and Daiichi Sankyo are going after.

Enhertu has already been approved for third-line use in HER2-positive breast cancer as well as previously-treated HER2-posiitve gastric cancer, and analysts think peak sales could be north of $2 billion, if the ADC can move into earlier lines of therapy and other tumour types.

AZ will need sales to reach that level if it wants to recoup the $6.9 billion it committed to the product, including $1.3 billion upfront, when it licensed the ADC from Daiichi Sankyo in 2019.

Kadcyla was once tipped to become the go-to treatment HER2-posiitve breast cancer when first line HER2 drugs like Roche's Herceptin (trastuzumab) and Perjeta (pertuzumab) had failed, but failed to meet the mark in pivotal trials.

Regardless, sales have still topped $1 billion, buoyed recently by its approval as an adjuvant therapy for early breast cancer as well as later-stage use.

Like its rivals, San Diego-based Ambrx is hoping to move its ADC into earlier lines of HER2-postive breast cancer treatment, although that will require additional financing, according to its IPO prospectus.

Some of the proceeds of the IPO – which would value Ambrx at $725 million, according to Renaissance Capital – will also go towards funding phase 1 development of PSMA-targeting antibody ARX517 for prostate cancer and to bring a CD7-targeting ADC into the clinic for haematological cancers and solid tumours.

Ambrx was planning can IPO in 2014 but failed to push it through, opting instead to sell the company to a group of Chinese investors and pharma companies including Shanghai Fosun Pharma and WuXi Pharmatech. It raised $200 million in financing last November.

The company plans to list on the NYSE with the AMAM ticker.