Sutro Biopharma reeled in an $85.4 million series E that will advance some early-stage programs as well as its cancer pipeline, which includes a pair of antibody-drug conjugates (ADCs).

Of its ADC assets, STRO-001 is in phase 1 for lymphoma and multiple myeloma, and STR-002 is slated to enter the clinic for ovarian and endometrial cancer by early 2019. Sutro's also got some bispecific antibodies, more ADCs and cytokine derivatives in the pipeline—it's partnering on those assets with Merck, Celgene and EMD Serono.

The company's in-house assets are based on its cell-free protein synthesis and site-specific conjugation platforms, which allow Sutro to quickly evaluate a variety of protein structures and manufacture a "highly optimized single molecular species" instead of the "imprecisely conjugated antibodies" made using conventional cell-based approaches.

"With [the] ExpressCF+ [platform] we incorporate non-natural amino acids into specific positions on the generated antibody for site-specific conjugation of cytotoxins with a linker and warhead to enable consistent, stable, pinpoint placement of STRO-001's toxic payload," said Trevor Hallam, Ph.D., Sutro's chief scientific officer, in a statement. "This leads to highly efficient delivery of the cytotoxin to tumor cells. By contrast, earlier generations of ADCs can have unpredictable pharmacologic properties, resulting in the potential for suboptimal stability, compromised efficacy and poor tolerability for patients." 

The financing comes five years after the Bay Area biotech's $26 million series D round and brings its total raised to more than $175 million. Samsara BioCapital and Surveyor Capital led the series E, alongside Alta Partners, Amgen Ventures, Celgene Corporation, Lilly Ventures, Skyline Ventures and SV Health Investors and a handful of new investors.

A few ADCs are already on the market—including Roche's HER2-targeting breast cancer drug Kadcyla and Spectrum Pharma's non-Hodgkin lymphoma drug Zevalin—and a number of them are in development. 

In April, Swiss biotech ADC Therapeutics ditched a candidate for HER2-positive breast cancers due to toxicity issues in a phase 1 trial. It said its decision to pull the program would not impact its other ADC assets, which are in early clinical development for a range of solid and blood cancers. That same month, AbbVie ended an early-stage trial for its ADC designed for solid tumors.

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