Abstract B105: ACTR707: a novel T-cell therapy for the treatment of relapsed or refractory CD20+ B cell lymphoma in combination with rituximab

The Antibody-Coupled T-cell Receptor (ACTR) platform is an autologous engineered T-cell therapy developed to combine with tumor-targeting antibodies to exert potent antitumor immune responses and tumor cell killing. The ACTR construct is composed of the extracellular domain of CD16 (FCGR3A; high-affinity V158 variant) linked to CD3ζ signaling and costimulatory domains. ACTR-expressing T cells are universal, and can be flexibly paired with desired therapeutic antibodies to target tumor antigens. Unum’s most advanced clinical candidate, ACTR087, is currently in clinical testing in combination with rituximab for the treatment of CD20+ B cell lymphoma (NCT02776813). To expand upon the ACTR platform, over 90 different domain-modified variant ACTRs were generated and evaluated through rigorous in vitro and in vivo testing in combination with a broad range of tumor-targeting antibodies for use in hematologic and solid tumor indications. ACTR T-cell cytotoxicity, cytokine release and proliferation assays, combined with performance in an in vitro repeated stimulation “stress test,” led to the identification of several lead ACTR candidates. The final ACTR candidate was identified on the ability to mediate tumor regressions of aggressive CD20+ lymphomas and HER2+ solid tumors in vivo when combined with rituximab or trastuzumab, respectively. These efforts identified ACTR707, an ACTR construct containing a CD28 costimulatory domain with additional molecular elements. Here we present nonclinical data supporting an ongoing first-in-human phase 1 clinical trial of ACTR707 in combination with rituximab. When combined with rituximab, ACTR707 T cells had potent activation, proliferation, cytokine production, and tumor-directed cytotoxicity in the presence of CD20+ lymphoma cell lines. ACTR707 T-cell in vitro activity was dependent on rituximab and was dose-titratable. ACTR707 T cells had potent antitumor efficacy in vivo in an aggressive Raji lymphoma xenograft model in NSG mice; ACTR707 T cell antitumor activity was ACTR T cell and rituximab dose-dependent, with higher T-cell numbers and antibody concentrations mediating improved responses. Further, ACTR707 in combination with rituximab outperformed a CD19 targeted CAR-T against aggressive Raji tumors in vivo. Taken together, these nonclinical data demonstrate the specificity and versatility of the ACTR707 T-cell therapeutic approach to target diverse cancer antigens and directly supports clinical testing in combination with rituximab. The phase I study, ATTCK-20-03 (NCT03189836), in subjects with relapsed or refractory CD20+ B cell lymphoma is a multicenter, single-arm, open-label dose escalation study evaluating the safety and antilymphoma efficacy of ACTR707 T cells in combination with rituximab. Subjects who meet eligibility requirements with histologically confirmed relapsed or refractory CD20+ B cell lymphoma of one of the following histologic subtypes are eligible: DLBCL, MCL, PMBCL, Gr3b-FL, TH-FL. The study’s primary objective is to assess the safety of the combination and define dose recommendations for further study. Additional study objectives include the assessment of antilymphoma activity, ACTR707 persistence, ACTR707 product characterization, inflammatory laboratory assessments, and rituximab pharmacokinetics. Enrollment is ongoing.

Citation Format: Greg Motz, Kathleen Whiteman, John Shin, Tapasya Pai, Casey Judge, Anthony Barnitz, James Hemphill, James Kim, Ann Ranger, Heather Huet, Kathleen McGinness, Birgit Schultes, Geoffrey Hodge, Michael Vasconcelles, Seth Ettenberg. ACTR707: a novel T-cell therapy for the treatment of relapsed or refractory CD20+ B cell lymphoma in combination with rituximab [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 B105.

Genetic Code Expansion in Zebrafish Embryos and Its Application to Optical Control of Cell Signaling

Site-specific incorporation of unnatural amino acids into proteins provides a powerful tool to study protein function. Here we report genetic code expansion in zebrafish embryos and its application to the optogenetic control of cell signaling. We genetically encoded four unnatural amino acids with a diverse set of functional groups, which included a photocaged lysine that was applied to the light-activation of luciferase and kinase activity. This approach enables versatile manipulation of protein function in live zebrafish embryos, a transparent and commonly used model organism to study embryonic development.

Optically Controlled Signal Amplification for DNA Computation

The hybridization chain reaction (HCR) and fuel-catalyst cycles have been applied to address the problem of signal amplification in DNA-based computation circuits. While they function efficiently, these signal amplifiers cannot be switched ON or OFF quickly and noninvasively. To overcome these limitations, a light-activated initiator strand for the HCR, which enabled fast optical OFF → ON switching, was developed. Similarly, when a light-activated version of the catalyst strand or the inhibitor strand of a fuel-catalyst cycle was applied, the cycle could be optically switched from OFF → ON or ON → OFF, respectively. To move the capabilities of these devices beyond solution-based operations, the components were embedded in agarose gels. Irradiation with customizable light patterns and at different time points demonstrated both spatial and temporal control. The addition of a translator gate enabled a spatially activated signal to travel along a predefined path, akin to a chemical wire. Overall, the addition of small light-cleavable photocaging groups to DNA signal amplification circuits enabled conditional control as well as fast photocontrol of signal amplification.

Optical Control of CRISPR/Cas9 Gene Editing

The CRISPR/Cas9 system has emerged as an important tool in biomedical research for a wide range of applications, with significant potential for genome engineering and gene therapy. In order to achieve conditional control of the CRISPR/Cas9 system, a genetically encoded light-activated Cas9 was engineered through the site-specific installation of a caged lysine amino acid. Several potential lysine residues were identified as viable caging sites that can be modified to optically control Cas9 function, as demonstrated through optical activation and deactivation of both exogenous and endogenous gene function.

Conditional control of alternative splicing through light-triggered splice-switching oligonucleotides

The spliceosome machinery is composed of several proteins and multiple small RNA molecules that are involved in gene regulation through the removal of introns from pre-mRNAs in order to assemble exon-based mRNA containing protein-coding sequences. Splice-switching oligonucleotides (SSOs) are genetic control elements that can be used to specifically control the expression of genes through correction of aberrant splicing pathways. A current limitation with SSO methodologies is the inability to achieve conditional control of their function paired with high spatial and temporal resolution. We addressed this limitation through site-specific installation of light-removable nucleobase-caging groups as well as photocleavable backbone linkers into synthetic SSOs. This enables optochemical OFF → ON and ON → OFF switching of their activity and thus precise control of alternative splicing. The use of light as a regulatory element allows for tight spatial and temporal control of splice switching in mammalian cells and animals.

Site-specific promoter caging enables optochemical gene activation in cells and animals

In cell and molecular biology, double-stranded circular DNA constructs, known as plasmids, are extensively used to express a gene of interest. These gene expression systems rely on a specific promoter region to drive the transcription of genes either constitutively (i.e., in a continually “ON” state) or conditionally (i.e., in response to a specific transcription initiator). However, controlling plasmid-based expression with high spatial and temporal resolution in cellular environments and in multicellular organisms remains challenging. To overcome this limitation, we have site-specifically installed nucleobase-caging groups within a plasmid promoter region to enable optochemical control of transcription and, thus, gene expression, via photolysis of the caging groups. Through the light-responsive modification of plasmid-based gene expression systems, we have demonstrated optochemical activation of an exogenous fluorescent reporter gene in both tissue culture and a live animal model, as well as light-induced overexpression of an endogenous signaling protein.

MicroRNA targeting of CoREST controls polarization of migrating cortical neurons

The migration of cortical projection neurons is a multistep process characterized by dynamic cell shape remodeling. The molecular basis of these changes remains elusive, and the present work describes how microRNAs (miRNAs) control neuronal polarization during radial migration. We show that miR-22 and miR-124 are expressed in the cortical wall where they target components of the CoREST/REST transcriptional repressor complex, thereby regulating doublecortin transcription in migrating neurons. This molecular pathway underlies radial migration by promoting dynamic multipolar-bipolar cell conversion at early phases of migration, and later stabilization of cell polarity to support locomotion on radial glia fibers. Thus, our work emphasizes key roles of some miRNAs that control radial migration during cerebral corticogenesis.

Genetically encoded light-activated transcription for spatiotemporal control of gene expression and gene silencing in mammalian cells

Photocaging provides a method to spatially and temporally control biological function and gene expression with high resolution. Proteins can be photochemically controlled through the site-specific installation of caging groups on amino acid side chains that are essential for protein function. The photocaging of a synthetic gene network using unnatural amino acid mutagenesis in mammalian cells was demonstrated with an engineered bacteriophage RNA polymerase. A caged T7 RNA polymerase was expressed in cells with an expanded genetic code and used in the photochemical activation of genes under control of an orthogonal T7 promoter, demonstrating tight spatial and temporal control. The synthetic gene expression system was validated with two reporter genes (luciferase and EGFP) and applied to the light-triggered transcription of short hairpin RNA constructs for the induction of RNA interference.

Spatiotemporal control of microRNA function using light-activated antagomirs

MicroRNAs (miRNAs) are small non-coding RNAs that act as post-transcriptional gene regulators and have been shown to regulate many biological processes including embryonal development, cell differentiation, apoptosis, and proliferation. Variations in the expression of certain miRNAs have been linked to a wide range of human diseases - especially cancer - and the diversity of miRNA targets suggests that they are involved in various cellular networks. Several tools have been developed to control the function of individual miRNAs and have been applied to study their biogenesis, biological role, and therapeutic potential; however, common methods lack a precise level of control that allows for the study of miRNA function with high spatial and temporal resolution. Light-activated miRNA antagomirs for mature miR-122 and miR-21 were developed through the site-specific installation of caging groups on the bases of selected nucleotides. Installation of caged nucleotides led to complete inhibition of the antagomir-miRNA hybridization and thus inactivation of antagomir function. The miRNA-inhibitory activity of the caged antagomirs was fully restored upon decaging through a brief UV irradiation. The synthesized antagomirs were applied to the photochemical regulation of miRNA function in mammalian cells. Moreover, spatial control over antagomir activity was obtained in mammalian cells through localized UV exposure. The presented approach enables the precise regulation of miRNA function and miRNA networks with unprecedented spatial and temporal resolution using UV irradiation and can be extended to any miRNA of interest.

Regulation of transcription through light-activation and light-deactivation of triplex-forming oligonucleotides in mammalian cells

Triplex-forming oligonucleotides (TFOs) are efficient tools to regulate gene expression through the inhibition of transcription. Here, nucleobase-caging technology was applied to the temporal regulation of transcription through light-activated TFOs. Through site-specific incorporation of caged thymidine nucleotides, the TFO:DNA triplex formation is blocked, rendering the TFO inactive. However, after a brief UV irradiation, the caging groups are removed, activating the TFO and leading to the inhibition of transcription. Furthermore, the synthesis and site-specific incorporation of caged deoxycytidine nucleotides within TFO inhibitor sequences was developed, allowing for the light-deactivation of TFO function and thus photochemical activation of gene expression. After UV-induced removal of the caging groups, the TFO forms a DNA dumbbell structure, rendering it inactive, releasing it from the DNA, and activating transcription. These are the first examples of light-regulated TFOs and their application in the photochemical activation and deactivation of gene expression. In addition, hairpin loop structures were found to significantly increase the efficacy of phosphodiester DNA-based TFOs in tissue culture.

DNA computation: a photochemically controlled AND gate

DNA computation is an emerging field that enables the assembly of complex circuits based on defined DNA logic gates. DNA-based logic gates have previously been operated through purely chemical means, controlling logic operations through DNA strands or other biomolecules. Although gates can operate through this manner, it limits temporal and spatial control of DNA-based logic operations. A photochemically controlled AND gate was developed through the incorporation of caged thymidine nucleotides into a DNA-based logic gate. By using light as the logic inputs, both spatial control and temporal control were achieved. In addition, design rules for light-regulated DNA logic gates were derived. A step-response, which can be found in a controller, was demonstrated. Photochemical inputs close the gap between DNA computation and silicon-based electrical circuitry, since light waves can be directly converted into electrical output signals and vice versa. This connection is important for the further development of an interface between DNA logic gates and electronic devices, enabling the connection of biological systems with electrical circuits.