Based on a technology that expands the genetic code of E. coli, Ambrx was founded in 2003. The technology was created in the laboratory of Peter Schultz, Ph.D., a chemist at Scripps Research Institute, who started the firm along with Richard DiMarchi, Ph.D., and Troy Wilson, Ph.D. (Dr. DiMarchi, now at Indiana University in Bloomington, co-developed the recombinant insulin Humalog while at Lilly Research Laboratories.)
Ambrx researchers improved Schultz’ prokaryotic process and named it ReCODE™ (reconstituting chemically orthogonal directed engineering). ReCODE combines medicinal chemistry with recombinant DNA-based protein biosynthesis to make next-generation protein therapeutics.
The technology incorporates a novel (non-natural) amino acid at a specific site into proteins like growth hormone. Then polyethylene glycol (PEG) is attached to enhance the protein’s pharmaceutical activity. The PEGylated protein remains active much longer than the native protein.
“We can do once-weekly or even once-monthly dosing compared to wild-type proteins that need daily administration,” says Ho Sung Cho, Ph.D., chief technology officer.
ReCODE improves on first-generation protein therapeutics, such as growth hormone, interferon-alpha, and interferon-beta, Dr. Cho remarks. These proteins are composed of only the 20 natural amino acids and all require frequent injections.
Second-generation protein therapeutics, in which PEG is attached to lysine or cysteine using conventional conjugation, require fewer injections, but they are heterogenous mixtures with suboptimal potency compared to the natural protein.
Ambrx’ ReCODE technology incorporates a novel Ambrx amino acid into a site that preserves protein function and allows site-specific attachment of PEG, explains Dr. Cho. Through directed evolution and selection, specialized orthogonal tRNA synthetases are evolved to selectively modify a similar orthogonal amber stop codon suppressing tRNA. The cell’s translational machinery then incorporates the Ambrx amino acid into a peptide sequence at positions controlled by the amber codon.
“The amber stop codon allows us to expand the genetic code beyond the natural 20 amino acids and code for amino acids of interest,” explains Dr. Cho.
Using ReCODE, Ambrx researchers have incorporated more than 50 new amino acids into proteins in E. coli, yeast, and mammalian cell systems. Ambrx’ method can be applied to multiple protein classes, including cytokines, peptides, and antibodies, to develop treatments for cancer, endocrine disorders, inflammation, and infectious diseases, Dr. Cho claims.
Bristol-Myers Squibb and Ambrx are collaborating to test two ReCODE proteins—Fibroblast Growth Factor 21 (FGF-21) and Relaxin. FGF-21 holds potential for treating type 2 diabetes, and the lead compound in this program, ARX618 (or PEG-FGF-21) is in the final stages of preclinical development. The hormone Relaxin, which plays a role in pregnancy and childbirth, may also improve cardiac function to prevent heart failure.
Another ReCODE product, ARX201, is a sustained action form of human growth hormone. ARX201 showed positive results in a Phase I/II study for patients with adult hormone deficiency. ARX201 required only weekly dosing. Levels of insulin-like growth factor-1, a marker for growth hormone activity, were restored to normal in 22 patients who received weekly injections of ARX201 for up to 26 weeks.
The newest extension of ReCODE is called EuCODE™, which uses eukaryotic cells to produce proteins in mammalian cell cultures like traditional CHO cells. “This allows us to create antibodies containing our novel amino acids,” says Dr. Cho. The antibodies are a key part of the company’s antibody drug conjugates (ADCs) platform. Ambrx scientists typically insert an amino acid into both heavy chains of an antibody, explains Dr. Cho. These amino acids then become the attachment site for drug conjugates.
For example, Ambrx created the amino acid para-acetylphenylalanine, a relative of natural phenylalanine, but with an added ketone functional group. The EuCODE system incorporates para-acetylphenylalanine into an antibody at a preselected site. “Then we functionalize a linker payload to react with the novel ketone group of the antibody,” says Dr. Cho. All of Ambrx’ ADCs are designed to preserve other important antibody functions, such as the ability to bind efficiently to antigens and complement.
Other methods for making ADCs imprecisely link molecules, resulting in heterogenous mixtures of ADCs with compromised efficacy and safety properties, notes Dr. Cho. The EuCODE technology may overcome the limitations of current types of ADCs by precisely linking the antibody and drug in a controlled manner to generate potentially best-in-class homogenous ADCs.
Merck recently began a collaboration with Ambrx to design and develop rationally optimized ADCs. The agreement combines Ambrx’ expertise in site-specific protein conjugation chemistry with Merck’s antibody and small molecule libraries. The combination has the potential to build a new family of biologic drug conjugates that selectively deliver small molecules to their site of action to treat oncology and other important diseases, according to Dr. Cho.
Many other ADCs are undergoing preclinical evaluation at Ambrx. “We have robust preclinical datasets showing that our ADCs are active against multiple tumor types in rodent models,” says Dr. Cho. Ambrx will advance some ADCs into clinical trials itself, and is looking for partners to develop other ADCs in its pipeline.