Expression of knotted1 ( kn1) and ZmLEC1, a maize homologue of the Arabidopsis LEAFY COTYLEDON1 ( LEC1) was studied using in situ hybridization during in vitro somatic embryogenesis of maize ( Zea mays L.) genotype Hi-II. Expression of kn1 was initially detected in a small group of cells (5-10) in the somatic embryo proper at the globular stage, in a specific region where the shoot meristem is initiating at the scutellar stage, and specifically in the shoot meristem at the coleoptilar stage. Expression of ZmLEC1 was strongly detected in the entire somatic embryo proper at the globular stage, gradually less in the differentiating scutellum at the scutellar and coleoptilar stages. The results of analyses show that the expression pattern of kn1 during in vitro somatic embryogenesis of maize is similar to that of kn1 observed during zygotic embryo development in maize. The expression pattern of ZmLEC1 in maize during in vitro development is similar to that of LEC1 in Arabidopsis during zygotic embryo development. These observations indicate that in vitro somatic embryogenesis likely proceeds through similar developmental pathways as zygotic embryo development, after somatic cells acquire competence to form embryos. In addition, based on the ZmLEC1 expression pattern, we suggest that expression of ZmLEC1 can be used as a reliable molecular marker for detecting early-stage in vitro somatic embryogenesis in maize.

Lateral gene transfer (LGT) from prokaryotes to microbial eukaryotes is usually detected by chance through genome-sequencing projects. Here, we explore a different, hypothesis-driven approach. We show that the fitness advantage associated with the transferred gene, typically invoked only in retrospect, can be used to design a functional screen capable of identifying postulated LGT cases. We hypothesized that beta-glucuronidase (gus) genes may be prone to LGT from bacteria to fungi (thought to lack gus) because this would enable fungi to utilize glucuronides in vertebrate urine as a carbon source. Using an enrichment procedure based on a glucose-releasing glucuronide analog (cellobiouronic acid), we isolated two gus(+) ascomycete fungi from soils (Penicillium canescens and Scopulariopsis sp.). A phylogenetic analysis suggested that their gus genes, as well as the gus genes identified in genomic sequences of the ascomycetes Aspergillus nidulans and Gibberella zeae, had been introgressed laterally from high-GC gram(+) bacteria. Two such bacteria (Arthrobacter spp.), isolated together with the gus(+) fungi, appeared to be the descendants of a bacterial donor organism from which gus had been transferred to fungi. This scenario was independently supported by similar substrate affinities of the encoded beta-glucuronidases, the absence of introns from fungal gus genes, and the similarity between the signal peptide-encoding 5' extensions of some fungal gus genes and the Arthrobacter sequences upstream of gus. Differences in the sequences of the fungal 5' extensions suggested at least two separate introgression events after the divergence of the two main Euascomycete classes. We suggest that deposition of glucuronides on soils as a result of the colonization of land by vertebrates may have favored LGT of gus from bacteria to fungi in soils.

T cell-engaging bispecific antibodies (TCBs) are highly potent therapeutics that direct the activity of cytotoxic T cells to tumors. TCBs have shown clinical activity in hematologic malignancies, but development of TCBs for solid tumor indications is proving more challenging. Due to their high potency, TCBs can target normal tissues with low antigen expression, resulting in significant on-target, off-tumor toxicity that can limit dosing to low levels. As a result, it has been difficult to reach the level of drug exposure required for efficacy without excessive toxicity. Therefore, novel methods are needed to enable the potent antitumor activity of TCBs while minimizing toxicity due to cytokine release and damage to healthy tissues. CytomX has developed a new class of recombinant, proteolytically activated antibody prodrugs (ProbodyTM therapeutics) that are “masked” to prevent binding to antigen in healthy tissue, but can become “unmasked” by proteases that are preferentially activated in the tumor microenvironment. In this way, Probody therapeutics are designed to increase therapeutic index by maximizing efficacy and minimizing on-target toxicity in normal tissues. Here we describe a T cell-engaging Bispecific Probody therapeutic (Pb-TCB) targeting Epidermal Growth Factor Receptor (EGFR) and CD3 that has been optimized for affinity, effector function, masking, and cleavability. In vitro, under protease-deficient conditions, we demonstrate that the unmasked EGFR-CD3 TCB has potent, EGFR-dependent tumor cell killing, while the doubly-masked EGFR-CD3 Pb-TCB reduces target-dependent cytotoxicity by more than 100,000-fold. However, in established tumor models where tumor-resident proteases are expected to be active, we demonstrate that Pb-TCBs potently induce tumor regressions. In nonhuman primates, the maximum tolerated dose (MTD) of the EGFR-CD3 Pb-TCB is more than 60-fold higher than the MTD of the unmasked TCB, and the tolerated exposure (AUC) is more than 10,000-fold higher. Finally, despite the 60-fold dose differential at the MTDs, transient serum cytokine and AST/ALT increases observed in nonhuman primates treated with the Pb-TCB are still lower than those induced by the TCB. By localizing activity to the tumor microenvironment, Pb-TCBs have the potential to expand clinical opportunities for T cell-engaging bispecific therapies that are limited by on-target toxicities, especially in solid tumors. Moreover, an EGFR-CD3 Pb-TCB has the potential to address EGFR-expressing tumors that are poorly responsive to existing EGFR-directed therapies. PROBODY is a trademark of CytomX Therapeutics, Inc. Citation Format: Leila M. Boustany, Laurie Wong, Clayton W. White, Linnea Diep, Yuanhui Huang, Shouchun Liu, Jennifer H. Richardson, W. Michael Kavanaugh, Bryan A. Irving. EGFR-CD3 bispecific Probody™ therapeutic induces tumor regressions and increases maximum tolerated dose >60-fold in preclinical studies [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A164.

T cell–engaging bispecific antibodies (TCB) are highly potent therapeutics that can recruit and activate cytotoxic T cells to stimulate an antitumor immune response. However, the development of TCBs against solid tumors has been limited by significant on-target toxicity to normal tissues. Probody therapeutics have been developed as a novel class of recombinant, protease-activated antibody prodrugs that are “masked” to reduce antigen binding in healthy tissues but can become conditionally unmasked by proteases that are preferentially active in the tumor microenvironment (TME). Here, we describe the preclinical efficacy and safety of CI107, a Probody TCB targeting EGFR and CD3. In vitro, the protease-activated, unmasked CI107 effectively bound EGFR and CD3 expressed on the surface of cells and induced T-cell activation, cytokine release, and cytotoxicity toward tumor cells. In contrast, dually masked CI107 displayed a >500-fold reduction in antigen binding and >15,000-fold reduction in cytotoxic activity. In vivo, CI107 potently induced dose-dependent tumor regression of established colon cancer xenografts in mice engrafted with human peripheral blood mononuclear cells. Furthermore, the MTD of CI107 in cynomolgus monkeys was more than 60-fold higher than that of the unmasked TCB, and much lower levels of toxicity were observed in animals receiving CI107. Therefore, by localizing activity to the TME and thus limiting toxicity to normal tissues, this Probody TCB demonstrates the potential to expand clinical opportunities for TCBs as effective anticancer therapies for solid tumor indications. Significance: A conditionally active EGFR-CD3 T cell–engaging Probody therapeutic expands the safety window of bispecific antibodies while maintaining efficacy in preclinical solid tumor settings.

Immune checkpoint blockade therapies have been shown to induce potent and durable anti-tumor immunity in many cancer types. Nevertheless, not all patients benefit from immunotherapy, and immune-related adverse events remain a problem. Recently, it has been demonstrated that Antibody Drug Conjugates (ADCs) are not only capable of killing cancer cells but also can act to induce the immunogenic cell death of tumor cells as well as directly activate dendritic cells. These results provided a rationale to combine ADCs with immunotherapy to enhance the potential of immune checkpoint blockade therapies in a broader population of patients. CytomX Therapeutics has developed a new class of antibodies called Probody™ therapeutics, designed to widen the therapeutic window by minimizing binding to target in healthy tissue while being specifically activated in the tumor microenvironment (TME) by tumor-associated proteases. Probody technology has been evaluated in preclinical studies in several antibody formats, with efficacy and increased safety windows observed for Probody therapeutics targeting the PD-1 pathway, Probody drug conjugates (PDCs) targeting highly expressed tumor antigens, and T-cell engaging bispecific Probody therapeutics. Here we extend our evaluation of the Probody platform to the combination of CX-2009, an investigational PDC targeting human CD166, with an investigational Probody therapeutic targeting PD-1. To evaluate the anti-tumor activity of PDC CX-2009 in a syngeneic mouse model, human CD166 was overexpressed on the surface of the CT-26 murine colon carcinoma cell line. The combination treatment of CX-2009 with a surrogate mouse anti-PD-1 Probody molecule significantly inhibited tumor growth in human CD166 positive CT-26 tumor-bearing mice as compared to CX-2009 or anti-PD-1 Probody molecule alone. Tumor rejection is partially dependent on CD8+ T cells as illustrated by the evidence of a CD8+ memory T cell response in a re-challenge assay, and a reduced activity of CX-2009 alone or in combination with a mouse anti-PD-1 Probody molecule after CD8+ T cell depletion. The immunogenic potential of CX-2009 was further evaluated in multiple in vitro assays using human cancer cells and human PBMCs. In contrast to its cytotoxic activity towards CD166+ tumor cells, CX-2009 spares T cells and may enhance T cell priming. These preclinical data demonstrate the potential utility of a combination of PDC CX-2009 with a Probody therapeutic targeting the PD-1 pathway. Generally, these data highlight the potential to combine ADCs or PDCs with immune checkpoint blockade therapies. PROBODY is a trademark of CytomX Therapeutics, Inc. Citation Format: Erwan Le Scolan, Tiffany Tse, Michael Krimm, Will Garner, Hikmat Assi, Jennifer Razo, Laurie Wong, Kenneth Wong, Victoria Singson, Jennifer Leong, Linnea Diep, Jennifer Richardson, Siew Schleyer, Dylan Daniel, Marcia Belvin, Michael Kavanaugh. A probody drug conjugate targeting CD166 (ALCAM) enhances preclinical antitumor activity of a probody therapeutic targeting PD-1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3202.