Preclinical Characterization of ARX517, a Site-specific Stable PSMA-Targeted Antibody Drug Conjugate for Treatment of Metastatic Castration-Resistant Prostate Cancer.

Metastatic castration-resistant prostate cancer (mCRPC) is an advanced disease in which patients ultimately fail standard of care androgen-deprivation therapies and exhibit poor survival rates. The prostate-specific membrane antigen (PSMA) has been validated as a mCRPC tumor antigen with over-expression in tumors and low expression in healthy tissues. Using our proprietary technology for incorporating synthetic amino acids (SAAs) into proteins at selected sites, we have developed ARX517, an antibody drug conjugate (ADC) which is composed of a humanized anti-PSMA antibody site-specifically conjugated to a tubulin inhibitor at a drug-to-antibody ratio of 2. After binding PSMA, ARX517 is internalized and catabolized, leading to cytotoxic payload delivery and apoptosis. To minimize premature payload release and maximize delivery to tumor cells, ARX517 employs a non-cleavable PEG linker and stable oxime conjugation enabled via SAA protein incorporation to ensure its overall stability. In vitro studies demonstrate that ARX517 selectively induces cytotoxicity of PSMA-expressing tumor cell lines. ARX517 exhibited a long terminal half-life and high serum exposure in mice, and dose-dependent anti-tumor activity in both enzalutamide-sensitive and -resistant CDX and PDX prostate cancer models. Repeat dose toxicokinetic studies in non-human primates demonstrated ARX517 was tolerated at exposures well above therapeutic exposures in mouse pharmacology studies, indicating a wide therapeutic index. In summary, ARX517 inhibited tumor growth in diverse mCRPC models, demonstrated a tolerable safety profile in monkeys, and had a wide therapeutic index based on preclinical exposure data. Based on the encouraging preclinical data, ARX517 is currently being evaluated in a Phase 1 clinical trial ([NCT04662580]).

Prophylactic treatment with PEGylated bovine IFNλ3 effectively bridges the gap in vaccine-induced immunity against FMD in cattle.

Foot-and-mouth disease (FMD) is a vesicular disease of cloven-hoofed animals with devastating economic implications. The current FMD vaccine, routinely used in enzootic countries, requires at least 7 days to induce protection. However, FMD vaccination is typically not recommended for use in non-enzootic areas, underscoring the need to develop new fast-acting therapies for FMD control during outbreaks. Interferons (IFNs) are among the immune system's first line of defense against viral infections. Bovine type III IFN delivered by a replication defective adenovirus (Ad) vector has effectively blocked FMD in cattle. However, the limited duration of protection-usually only 1-3 days post-treatment (dpt)-diminishes its utility as a field therapeutic. Here, we test whether polyethylene glycosylation (PEGylation) of recombinant bovine IFNλ3 (PEGboIFNλ3) can extend the duration of IFN-induced prevention of FMDV infection in both vaccinated and unvaccinated cattle. We treated groups of heifers with PEGboIFNλ3 alone or in combination with an adenovirus-based FMD O1Manisa vaccine (Adt-O1M) at either 3 or 5 days prior to challenge with homologous wild type FMDV. We found that pre-treatment with PEGboIFNλ3 was highly effective at preventing clinical FMD when administered at either time point, with or without co-administration of Adt-O1M vaccine. PEGboIFNλ3 protein was detectable systemically for >10 days and antiviral activity for 4 days following administration. Furthermore, in combination with Adt-O1M vaccine, we observed a strong induction of FMDV-specific IFNγ+ T cell response, demonstrating its adjuvanticity when co-administered with a vaccine. Our results demonstrate the promise of this modified IFN as a pre-exposure prophylactic therapy for use in emergency outbreak scenarios.

Analytical comparability to evaluate impact of manufacturing changes of ARX788, an Anti-HER2 ADC in late-stage clinical development.

ARX788 is an anti-HER2 antibody drug conjugate (ADC) developed using Ambrx proprietary Engineered Precision Biologics technology. The manufacturing process of ARX788 has been optimized during the course of early to late-phase clinical development. A comprehensive evaluation of side-by-side comparability between pre- and post-change process for ARX788 drug substance and drug product from a quality perspective was conducted based on ICH Q5E guidelines consisting of batch release assays, physicochemical and biophysical characterization, biological characterization, and forced degradation studies. All results have substantiated a high degree of similarity between the pre- and post-change ARX788 drug substance batches and drug product lots, demonstrating that the process manufacturing changes did not impact product quality.

ARX788, a Site-specific Anti-HER2 Antibody-Drug Conjugate, Demonstrates Potent and Selective Activity in HER2-low and T-DM1-resistant Breast and Gastric Cancers.

First-generation antibody-drug conjugates (ADC) are heterogeneous mixtures that have shown clinical benefit, but generally exhibited safety issues and a narrow therapeutic window due, in part, to off-target toxicity caused by ADC instability. ARX788 is a next-generation, site-specific anti-HER2 ADC that utilizes a unique nonnatural amino acid-enabled conjugation technology and a noncleavable Amberstatin (AS269) drug-linker to generate a homogeneous ADC with a drug-to-antibody ratio of 1.9. ARX788 exhibits high serum stability in mice and a relatively long ADC half-life of 12.5 days. When compared in vitro against T-DM1 across a panel of cancer cell lines, ARX788 showed superior activity in the lower HER2-expressing cell lines and no activity in normal cardiomyocyte cells. Similarly, ARX788 significantly inhibited tumor growth, and generally outperformed T-DM1 in HER2-high and HER2-low expression xenograft models. Breast and gastric cancer patient-derived xenograft studies confirmed strong antitumor activity of ARX788 in HER2-positive and HER2-low expression tumors, as well as in a T-DM1-resistant model. The encouraging preclinical data support the further development of ARX788 for treatment of patients with HER2-positive breast and gastric cancer, including those who have developed T-DM1 resistance, and patients with HER2-low expression tumors who are currently ineligible to receive HER2-targeted therapy.

Novel Phosphate Modified Cathepsin B Linkers: Improving Aqueous Solubility and Enhancing Payload Scope of ADCs.

In an effort to examine the utility of antibody-drug conjugates (ADCs) beyond oncology indications, a novel phosphate bridged Cathepsin B sensitive linker was developed to enable the targeted delivery of glucocorticoids. Phosphate bridging of the Cathepsin B sensitive linkers allows for payload attachment at an aliphatic alcohol. As small molecule drug-linkers, these aqueous soluble phosphate containing drug-linkers were found to have robust plasma stability coupled with rapid release of payload in a lysosomal environment. Site-specific ADCs were successfully made between these drug-linkers and an antibody against human CD70, a receptor specifically expressed in immune cells but also found aberrantly expressed in multiple human carcinomas. These ADCs demonstrated in vitro targeted delivery of glucocorticoids to a representative cell line as measured by changes in glucocorticoid receptor (GR) mediated gene mRNA levels. This novel linker expands the scope of potential ADC payloads by allowing an aliphatic alcohol to be a stable, yet cleavable attachment site. This phosphate linker may have broad utility for internalizing ADCs as well as other targeted delivery platforms.

Discovery of Pyrophosphate Diesters as Tunable, Soluble, and Bioorthogonal Linkers for Site-Specific Antibody-Drug Conjugates.

As part of an effort to examine the utility of antibody-drug conjugates (ADCs) beyond oncology indications, a novel pyrophosphate ester linker was discovered to enable the targeted delivery of glucocorticoids. As small molecules, these highly soluble phosphate ester drug linkers were found to have ideal orthogonal properties: robust plasma stability coupled with rapid release of payload in a lysosomal environment. Building upon these findings, site-specific ADCs were made between this drug linker combination and an antibody against human CD70, a receptor specifically expressed in immune cells but also found aberrantly expressed in multiple human carcinomas. Full characterization of these ADCs enabled procession to in vitro proof of concept, wherein ADCs 1-22 and 1-37 were demonstrated to afford potent, targeted delivery of glucocorticoids to a representative cell line, as measured by changes in glucocorticoid receptor-mediated gene mRNA levels. These activities were found to be antibody-, linker-, and payload-dependent. Preliminary mechanistic studies support the notion that lysosomal trafficking and enzymatic linker cleavage are required for activity and that the utility for the pyrophosphate linker may be general for internalizing ADCs as well as other targeted delivery platforms.

In vitro and in vivo evaluation of cysteine and site specific conjugated herceptin antibody-drug conjugates.

Antibody drug conjugates (ADCs) are monoclonal antibodies designed to deliver a cytotoxic drug selectively to antigen expressing cells. Several components of an ADC including the selection of the antibody, the linker, the cytotoxic drug payload and the site of attachment used to attach the drug to the antibody are critical to the activity and development of the ADC. The cytotoxic drugs or payloads used to make ADCs are typically conjugated to the antibody through cysteine or lysine residues. This results in ADCs that have a heterogeneous number of drugs per antibody. The number of drugs per antibody commonly referred to as the drug to antibody ratio (DAR), can vary between 0 and 8 drugs for a IgG1 antibody. Antibodies with 0 drugs are ineffective and compete with the ADC for binding to the antigen expressing cells. Antibodies with 8 drugs per antibody have reduced in vivo stability, which may contribute to non target related toxicities. In these studies we incorporated a non-natural amino acid, para acetyl phenylalanine, at two unique sites within an antibody against Her2/neu. We covalently attached a cytotoxic drug to these sites to form an ADC which contains two drugs per antibody. We report the results from the first direct preclinical comparison of a site specific non-natural amino acid anti-Her2 ADC and a cysteine conjugated anti-Her2 ADC. We report that the site specific non-natural amino acid anti-Her2 ADCs have superior in vitro serum stability and preclinical toxicology profile in rats as compared to the cysteine conjugated anti-Her2 ADCs. We also demonstrate that the site specific non-natural amino acid anti-Her2 ADCs maintain their in vitro potency and in vivo efficacy against Her2 expressing human tumor cell lines. Our data suggests that site specific non-natural amino acid ADCs may have a superior therapeutic window than cysteine conjugated ADCs.

Optimized clinical performance of growth hormone with an expanded genetic code.

The ribosomal incorporation of nonnative amino acids into polypeptides in living cells provides the opportunity to endow therapeutic proteins with unique pharmacological properties. We report here the first clinical study of a biosynthetic protein produced using an expanded genetic code. Incorporation of p-acetylphenylalanine (pAcF) at distinct locations in human growth hormone (hGH) allowed site-specific conjugation with polyethylene glycol (PEG) to produce homogeneous hGH variants. A mono-PEGylated mutant hGH modified at residue 35 demonstrated favorable pharmacodynamic properties in GH-deficient rats. Clinical studies in GH-deficient adults demonstrated efficacy and safety comparable to native human growth hormone therapy but with increased potency and reduced injection frequency. This example illustrates the utility of nonnative amino acids to optimize protein therapeutics in an analogous fashion to the use of medicinal chemistry to optimize conventional natural products, low molecular weight drugs, and peptides.