Induction of TRPC6 channel in acquired forms of proteinuric kidney disease

Injury to podocytes and their slit diaphragms typically leads to marked proteinuria. Mutations in the TRPC6 gene that codes for a slit diaphragm-associated, cation-permeable ion channel have been shown recently to co-segregate with hereditary forms of progressive kidney failure. Herein is shown that induced expression of wild-type TRPC6 is a common feature of human proteinuric kidney diseases, with highest induction observed in membranous nephropathy. Cultured podocytes that are exposed to complement upregulate TRPC6 protein. Stimulation of receptor-operated channels in puromycin aminonucleoside-treated podocytes leads to increased calcium influx in a time- and dosage-dependent manner. Mechanistically, it is shown that TRPC6 is functionally connected to the podocyte actin cytoskeleton, which is rearranged upon overexpression of TRPC6. Transient in vivo gene delivery of TRPC6 into mice leads to expression of TRPC6 protein at the slit diaphragm and causes proteinuria. These studies suggest the involvement of TRPC6 in the pathology of nongenetic forms of proteinuric disease.

BMP7 acts in murine lens placode development

Targeted inactivation of the Bmp7 gene in mouse leads to eye defects with late onset and variable penetrance (A. T. Dudley et al., 1995, Genes Dev. 9, 2795-2807; G. Luo et al., 1995, Genes Dev. 9, 2808-2820). Here we report that the expressivity of the Bmp7 mutant phenotype markedly increases in a C3H/He genetic background and that the phenotype implicates Bmp7 in the early stages of lens development. Immunolocalization experiments show that BMP7 protein is present in the head ectoderm at the time of lens placode induction. Using an in vitro culture system, we demonstrate that addition of BMP7 antagonists during the period of lens placode induction inhibits lens formation, indicating a role for BMP7 in lens placode development. Next, to integrate Bmp7 into a developmental pathway controlling formation of the lens placode, we examined the expression of several early lens placode-specific markers in Bmp7 mutant embryos. In these embryos, Pax6 head ectoderm expression is lost just prior to the time when the lens placode should appear, while in Pax6-deficient (Sey/Sey) embryos, Bmp7 expression is maintained. These results could suggest a simple linear pathway in placode induction in which Bmp7 functions upstream of Pax6 and regulates lens placode induction. At odds with this interpretation, however, is the finding that expression of secreted Frizzled Related Protein-2 (sFRP-2), a component of the Wnt signaling pathway which is expressed in prospective lens placode, is absent in Sey/Sey embryos but initially present in Bmp7 mutants. This suggests a different model in which Bmp7 function is required to maintain Pax6 expression after induction, during a preplacodal stage of lens development. We conclude that Bmp7 is a critical component of the genetic mechanism(s) controlling lens placode formation.

Selective inhibition of TGFβ1 activation overcomes primary resistance to checkpoint blockade therapy by altering tumor immune landscape

Despite breakthroughs achieved with cancer checkpoint blockade therapy (CBT), many patients do not respond to anti-programmed cell death-1 (PD-1) due to primary or acquired resistance. Human tumor profiling and preclinical studies in tumor models have recently uncovered transforming growth factor-β (TGFβ) signaling activity as a potential point of intervention to overcome primary resistance to CBT. However, the development of therapies targeting TGFβ signaling has been hindered by dose-limiting cardiotoxicities, possibly due to nonselective inhibition of multiple TGFβ isoforms. Analysis of mRNA expression data from The Cancer Genome Atlas revealed that TGFΒ1 is the most prevalent TGFβ isoform expressed in many types of human tumors, suggesting that TGFβ1 may be a key contributor to primary CBT resistance. To test whether selective TGFβ1 inhibition is sufficient to overcome CBT resistance, we generated a high-affinity, fully human antibody, SRK-181, that selectively binds to latent TGFβ1 and inhibits its activation. Coadministration of SRK-181-mIgG1 and an anti-PD-1 antibody in mice harboring syngeneic tumors refractory to anti-PD-1 treatment induced profound antitumor responses and survival benefit. Specific targeting of TGFβ1 was also effective in tumors expressing more than one TGFβ isoform. Combined SRK-181-mIgG1 and anti-PD-1 treatment resulted in increased intratumoral CD8⁺ T cells and decreased immunosuppressive myeloid cells. No cardiac valvulopathy was observed in a 4-week rat toxicology study with SRK-181, suggesting that selectively blocking TGFβ1 activation may avoid dose-limiting toxicities previously observed with pan-TGFβ inhibitors. These results establish a rationale for exploring selective TGFβ1 inhibition to overcome primary resistance to CBT.

Vertebrate eye development as modeled in Drosophila

Pax6, a member of the paired-box family of transcription factors, is critical for oculogenesis in both vertebrates and insects. Identification of potential vertebrate Pax6 targets has been guided by studies in Drosophila, where the Pax6 homologs eyeless ( ey ) and twin of eyeless ( toy ) function within a network of genes that synergistically pattern the developing fly eye. These targets, which share homology with the fly genes sine oculis, eyes absent and dachshund, exist in mice and humans as the Six, Eya and Dach gene families. Members of these gene families are present in the developing vertebrate eye, and preliminary studies suggest that they may function in a network analogous to that in the fly. Thus, despite radically different architecture, a similar molecular scaffold underlies both vertebrate and fly eye patterning, suggesting that the considerable power of Drosophila genetics can be harnessed to study mammalian ocular development.

Expression pattern of the Kallmann syndrome gene in the olfactory system suggests a role in neuronal targeting

Kallmann syndrome is a genetic disorder characterized by a defect in olfactory system development, which appears to be due to an abnormality in the migration of olfactory axons and gonadotropin releasing hormone (Gn-RH) producing neurons. The X-linked Kallmann syndrome gene shares significant similarities with molecules involved in neural development. We have now isolated the evolutionarily conserved chicken homologue of the Kallmann gene. In the developing and adult chicken, high levels of expression were found in the mitral cells of the olfactory bulb (the target of olfactory axons) and in the Purkinje cells of the cerebellar cortex, both areas affected in patients with Kallmann syndrome. We propose a model in which the Kallmann syndrome gene product is a signal molecule required for neuronal targeting throughout life.

The expression pattern of the chick homeobox gene gMHox suggests a role in patterning of the limbs and face and in compartmentalization of somites

MHox is a homeodomain protein that binds an essential element in the core of the muscle creatine kinase enhancer. In the mouse embryo, MHox expression is restricted to mesenchymal cells; in adult mice the gene is highly expressed in skeletal and cardiac muscle. To further define the functions of MHox during embryogenesis, we have cloned its chicken homolog, termed gMHox, and analyzed its properties and detailed expression patterns. Our studies show that the amino acid sequence and DNA-binding properties of the avian and murine gene products are very similar. Furthermore, the sites of expression are alike with high levels of expression in the splanchnic mesoderm, in the somatic mesoderm, in the limb bud mesoderm, in the dermatome and in the dermis, and in the ectomesenchyme of the face. gMHox became downregulated as chondrogenesis proceeded, whereas its expression was maintained in perichondrium and undifferentiated mesenchymal cells beneath the surface ectoderm. Such a pattern of expression suggests that gMHox may participate in maintenance of mesenchymal cell lineages derived from both mesoderm and the neural crest and in patterning of the limbs and the face. Removal of the surface ectoderm overlying the somites has no visible effect on the architecture of somites but results in the failure of gMHox to be expressed in the underlying dermatome, suggesting that regulation of gMHox expression in these cells is dependent on cues emanating from the surface ectoderm.

Developmental regulation of the orphan receptor COUP-TF II gene in spinal motor neurons

Members of the steroid/thyroid hormone receptor superfamily are involved in the control of cell identity and of pattern formation during embryonic development. Chicken ovalbumin upstream promoter-transcription factors (COUP-TFs) can act as regulators of various steroid/thyroid hormone receptor pathways. To begin to study the role of COUP-TFs during embryogenesis, we cloned a chicken COUP-TF (cCOUP-TF II) which is highly homologous to human COUP-TF II. Northern analysis revealed high levels of cCOUP-TF II transcripts during organogenesis. Nuclear extracts from whole embryos and from embryonic spinal cords were used in electrophoretic mobility shift assays. These assays showed that COUP-TF protein is present in these tissues and is capable of binding to a COUP element (a direct repeat of AGGTCA with one base pair spacing). Analysis of cCOUP-TF expression by in situ hybridization revealed high levels of cCOUP-TF II mRNA in the developing spinal motor neurons. Since the ventral properties of the spinal cord, including the development of motor neurons, is in part established by inductive signals from the notochord, we transplanted an additional notochord next to the dorsal region of the neural tube in order to induce ectopic motor neurons. We observed that an ectopic notochord induced cCOUP-TF II gene expression in the dorsal spinal cord in a region coextensive with ectopic domains of SC1 and Islet-1, two previously identified motor neuron markers. Collectively, our studies raise the possibility that cCOUP-TF II is involved in motor neuron development.

Conditional BMP inhibition in Xenopus reveals stage-specific roles for BMPs in neural and neural crest induction

Bone morphogenetic protein (BMP) inhibition has been proposed as the primary determinant of neural cell fate in the developing Xenopus ectoderm. The evidence supporting this hypothesis comes from experiments in explanted "animal cap" ectoderm and in intact embryos using BMP antagonists that are unregulated and active well before gastrulation. While informative, these experiments cannot answer questions regarding the timing of signals and the behavior of cells in the more complex environment of the embryo. To examine the effects of BMP antagonism at defined times in intact embryos, we have generated a novel, two-component system for conditional BMP inhibition. We find that while blocking BMP signals induces ectopic neural tissue both in animal caps and in vivo, in intact embryos, it can only do so prior to late blastula stage (stage 9), well before the onset of gastrulation. Later inhibition does not induce neural identity, but does induce ectopic neural crest, suggesting that BMP antagonists play temporally distinct roles in establishing neural and neural crest identity. By combining BMP inhibition with fibroblast growth factor (FGF) activation, the neural inductive response in whole embryos is greatly enhanced and is no longer limited to pre-gastrula ectoderm. Thus, BMP inhibition during gastrulation is insufficient for neural induction in intact embryos, arguing against a BMP gradient as the sole determinant of ectodermal cell fate in the frog.

Expression of the Kallmann syndrome gene in human fetal brain and in the manipulated chick embryo

Kallmann syndrome is an inherited disorder characterized by an abnormality in olfactory system development. The gene for the X-linked form of this disorder (KAL) maps to Xp22.3 and encodes a protein sharing homologies with molecules involved in neuronal migration and axonal pathfinding. Here we report the expression pattern of the KAL gene in various parts of the human fetal brain. We found KAL transcripts in granule cells of the olfactory bulb and the cerebellum, in the dorsomedial thalamus and in the developing cerebral cortex. To determine whether or not signals from the olfactory nerve are required for KAL expression in the olfactory bulb, we analyzed chick embryos in which the olfactory placode was surgically removed. Those embryos lacking an olfactory nerve had a histologically abnormal bulb which nevertheless expressed the KAL gene at high levels. These findings indicate that, while the development of the proper cytoarchitecture of the olfactory bulb requires the innervation by olfactory axons, the expression of KAL is independent of such developmental processes.

Calcium mediates glomerular filtration through calcineurin and mTORC2/Akt signaling

Alterations to the structure of the glomerular filtration barrier lead to effacement of podocyte foot processes, leakage of albumin, and the development of proteinuria. To better understand the signaling pathways involved in the response of the glomerular filtration barrier to injury, we studied freshly isolated rat glomeruli, which allows for the monitoring and pharmacologic manipulation of early signaling events. Administration of protamine sulfate rapidly damaged the isolated glomeruli, resulting in foot process effacement and albumin leakage. Inhibition of calcium channels and chelation of extracellular calcium reduced protamine sulfate-induced damage, suggesting that calcium signaling plays a critical role in the initial stages of glomerular injury. Calcineurin inhibitors (FK506 and cyclosporine A) and the cathepsin L inhibitor E64 all inhibited protamine sulfate-mediated barrier changes, which suggests that calcium signaling acts, in part, through calcineurin- and cathepsin L-dependent cleavage of synaptopodin, a regulator of actin dynamics. The mTOR inhibitor rapamycin also protected glomeruli, demonstrating that calcium signaling has additional calcineurin-independent components. Furthermore, activation of Akt through mTOR had a direct role on glomerular barrier integrity, and activation of calcium channels mediated this process, likely independent of phosphoinositide 3-kinase. Taken together, these results demonstrate the importance of calcium and related signaling pathways in the structure and function of the glomerular filtration barrier.

Glomerulopathy in the KK.Cg-A(y) /J mouse reflects the pathology of diabetic nephropathy

The KK.Cg-A (y) /J (KK-A (y) ) mouse strain is a previously described model of type 2 diabetes with renal impairment. In the present study, female KK-A (y) mice received an elevated fat content diet (24% of calories), and a cohort was uninephrectomized (Unx) to drive renal disease severity. Compared to KK-a/a controls, 26-week-old KK-A (y) mice had elevated HbA1c, insulin, leptin, triglycerides, and cholesterol, and Unx further elevated these markers of metabolic dysregulation. Unx KK-A (y) mice also exhibited elevated serum BUN and reduced glomerular filtration, indicating that reduction in renal mass leads to more severe impairment in renal function. Glomerular hypertrophy and hypercellularity, mesangial matrix expansion, podocyte effacement, and basement membrane thickening were present in both binephric and uninephrectomized cohorts. Glomerular size was increased in both groups, but podocyte density was reduced only in the Unx animals. Consistent with functional and histological evidence of increased injury, fibrotic (fibronectin 1, MMP9, and TGF β 1) and inflammatory (IL-6, CD68) genes were markedly upregulated in Unx KK-A (y) mice, while podocyte markers (nephrin and podocin) were significantly decreased. These data suggest podocyte injury developing into glomerulopathy in KK-A (y) mice. The addition of uninephrectomy enhances renal injury in this model, resulting in a disease which more closely resembles human diabetic nephropathy.

CXA-10, a Nitrated Fatty Acid, Is Renoprotective in Deoxycorticosterone Acetate-Salt Nephropathy

Underlying pathogenic mechanisms in chronic kidney disease (CKD) include chronic inflammation, oxidant stress, and matrix remodeling associated with dysregulated nuclear factor-κ B, nuclear factor-κ B, and SMAD signaling pathways, respectively. Important cytoprotective mechanisms activated by oxidative inflammatory conditions are mediated by nitrated fatty acids that covalently modify proteins to limit inflammation and oxidant stress. In the present study, we evaluated the effects of chronic treatment with CXA-10 (10-nitro-9(E)-octadec-9-enoic acid) in the uninephrectomized deoxycorticosterone acetate-high-salt mouse model of CKD. After 4 weeks of treatment, CXA-10 [2.5 millligrams per kilogram (mpk), p.o.] significantly attenuated increases in plasma cholesterol, heart weight, and kidney weight observed in the model without impacting systemic arterial blood pressure. CXA-10 also reduced albuminuria, nephrinuria, glomerular hypertrophy, and glomerulosclerosis in the model. Inflammatory MCP-1 and fibrosis (collagen, fibronectin, plasminogen activator inhibitor-1, and osteopontin) renal biomarkers were significantly reduced in the CXA-10 (2.5 mpk) group. The anti-inflammatory and antifibrotic effects, as well as glomerular protection, were not observed in the enalapril-treated group. Also, CXA-10 appears to exhibit hormesis as all protective effects observed in the low-dose group were absent in the high-dose group (12.5 mpk). Taken together, these findings demonstrate that, at the appropriate dose, the nitrated fatty acid CXA-10 exhibits anti-inflammatory and antifibrotic effects in the kidney and limits renal injury in a model of CKD.

Development and Optimization of Bifunctional Fusion Proteins to Locally Modulate Complement Activation in Diseased Tissue

Sustained complement activation is an underlying pathologic driver in many inflammatory and autoimmune diseases. Currently approved anti-complement therapies are directed at the systemic blockade of complement. Consequently, these therapies provide widespread inhibition of complement pathway activity, beyond the site of ongoing activation and the intended pharmacodynamic (PD) effects. Given the essential role for complement in both innate and adaptive immunity, there is a need for therapies that inhibit complement in diseased tissue while limiting systemic blockade. One potential approach focuses on the development of novel fusion proteins that enable tissue-targeted delivery of complement negative regulatory proteins. These therapies are expected to provide increased potency and prolonged tissue PD, decreased dosing frequency, and the potential for improved safety profiles. We created a library of bifunctional fusion proteins that direct a fragment of the complement negative regulator, complement receptor type 1 (CR1) to sites of tissue injury. Tissue targeting is accomplished through the binding of the fusion protein to complement C3 fragments that contain a surface-exposed C3d domain and which are covalently deposited on tissues where complement is being activated. To that end, we generated a fusion protein that contains an anti-C3d monoclonal antibody recombinantly linked to the first 10 consensus repeats of CR1 (CR11-10) with the intention of delivering high local concentrations of this complement negative regulatory domain to tissue-bound complement C3 fragments iC3b, C3dg and C3d. Biochemical and in vitro characterization identified several fusion proteins that inhibit complement while maintaining the C3d domain binding properties of the parent monoclonal antibody. Preclinical in vivo studies further demonstrate that anti-C3d fusion proteins effectively distribute to injured tissue and reduce C3 fragment deposition for periods beyond 14 days. The in vitro and in vivo profiles support the further evaluation of C3d mAb-CR11-10 as a novel approach to restore proper complement activation in diseased tissue in the absence of continuous systemic complement blockade.

A Sensitive and Selective Immunoassay for the Quantitation of Serum Latent Myostatin after In Vivo Administration of SRK-015, a Selective Inhibitor of Myostatin Activation

Myostatin, a member of the transforming growth factor β (TGFβ) superfamily, is a key regulator of skeletal muscle mass and a therapeutic target for muscle wasting diseases. We developed a human monoclonal antibody, SRK-015, that selectively binds to and inhibits proteolytic processing of myostatin precursors, thereby preventing growth factor release from the latent complex. As a consequence of antibody binding, latent myostatin accumulates in the circulation of animals treated with SRK-015 or closely related antibodies, suggesting that quantitation of latent myostatin in serum may serve as a biomarker for target engagement. To accurately measure SRK-015 target engagement, we developed a sensitive plate-based electrochemiluminescent immunoassay to quantitate latent myostatin in serum samples. The assay selectively recognizes latent myostatin without cross-reactivity to promyostatin, mature myostatin, or closely related members of the TGFβ superfamily. To enable use of the assay in samples from animals dosed with SRK-015, we incorporated a low-pH step that dissociates SRK-015 from latent myostatin, improving drug tolerance of the assay. The assay meets inter- and intra-assay accuracy and precision acceptance criteria, and it has a lower limit of quantitation (LLOQ) of 10 ng/mL. We then tested serum samples from a pharmacology study in cynomolgus monkeys treated with SRK-015. Serum latent myostatin increases after treatment with SRK-015, reaches a dose-dependent plateau approximately 20 days after dosing, and trends back toward baseline after cessation of antibody dosing. Taken together, these data suggest that this assay can be used to accurately measure levels of the primary circulating form of myostatin in population-based or pharmacodynamic studies.

An antibody that inhibits TGF-β1 release from latent extracellular matrix complexes attenuates the progression of renal fibrosis

Inhibitors of the transforming growth factor-β (TGF-β) pathway are potentially promising antifibrotic therapies, but nonselective simultaneous inhibition of all three TGF-β homologs has safety liabilities. TGF-β1 is noncovalently bound to a latency-associated peptide that is, in turn, covalently bound to different presenting molecules within large latent complexes. The latent TGF-β-binding proteins (LTBPs) present TGF-β1 in the extracellular matrix, and TGF-β1 is presented on immune cells by two transmembrane proteins, glycoprotein A repetitions predominant (GARP) and leucine-rich repeat protein 33 (LRRC33). Here, we describe LTBP-49247, an antibody that selectively bound to and inhibited the activation of TGF-β1 presented by LTBPs but did not bind to TGF-β1 presented by GARP or LRRC33. Structural studies demonstrated that LTBP-49247 recognized an epitope on LTBP-presented TGF-β1 that is not accessible on GARP- or LRRC33-presented TGF-β1, explaining the antibody's selectivity for LTBP-complexed TGF-β1. In two rodent models of kidney fibrosis of different etiologies, LTBP-49247 attenuated fibrotic progression, indicating the central role of LTBP-presented TGF-β1 in renal fibrosis. In mice, LTBP-49247 did not have the toxic effects associated with less selective TGF-β inhibitors. These results establish the feasibility of selectively targeting LTBP-bound TGF-β1 as an approach for treating fibrosis.

Abstract 1801: Development of a comprehensive biomarker strategy to support phase 1 clinical trial of SRK-181 the latent TGFβ1 inhibitor

TGFβ signaling appears to be a key mediator of primary resistance to programmed cell death protein (PD-1) pathway blockade. SRK-181 is an investigational stage, high-affinity, fully human antibody that selectively binds to latent TGFβ1 and inhibits its activation. Preclinical data demonstrate that combining SRK-181 with a PD-1 inhibitor modulates tumor microenvironment (TME), including an influx of CD8 positive T cells that correlates with anti-tumor responses. The ongoing DRAGON trial is a multi-center, open-label, Phase 1, first in-human (FIH), dose-escalation, and dose-expansion study. The goal of the trial is to evaluate the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and efficacy of SRK-181 administered alone and in combination with an anti-PD-(L)-1 in adult patients with locally advanced or metastatic solid tumors. To support this ongoing DRAGON clinical trial and further explore the mechanism of action of SRK-181, a comprehensive biomarker strategy is being developed to assess the alternation of immune profile in TME and potential predictors of therapeutic response to SRK-181. Here we describe the development and refinement of several biomarker assays. First, an image analysis-based algorithm for CD8 immunohistochemistry (IHC) is established utilizing human cancer tissue in a pre-clinical study. This novel digital pathology analysis enables identification of CD8 positive T cells in discrete compartments, including the tumor nests, stroma and tumor/stromal margins, to better capture the heterogeneity of the CD8 signal within tissues. Second, we describe methods to evaluate the TGFβ pathway including quantitative analysis of tumor tissue phospho-Smad2 and circulatory levels of TGFβ1 ligand. A companion assay to exclude blood samples with nonspecific background signals has been characterized and will be performed in parallel when evaluating circulatory TGFβ1 in clinic. In summary, we present several novel, tailored biomarker readouts that are part of a broader biomarker strategy aimed at maximizing detection of relevant clinical data to both support the ongoing clinical trial and provide further insight into the mechanism of action of SRK-181.

Citation Format: Christopher Brueckner, Ryan Faucette, Charles Caldwell, Jr., Sofia Reitsma, Stefan Wawersik, Ashish Kalra, Lu Gan, Si Tuen Lee-Hoeflich. Development of a comprehensive biomarker strategy to support phase 1 clinical trial of SRK-181 the latent TGFβ1 inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1801.

Tissue-targeted regulators of complement for amelioration of human disease: rationale and novel therapeutic strategies

The complement system is an essential part of innate immunity, and dysregulated complement is an underlying driver in many inflammatory and autoimmune diseases. Currently approved complement-focused therapeutics rely on systemic blockade of complement activation, but a major challenge with this approach is that complement components exist in high abundance and undergo rapid systemic turnover, creating a large pharmacologic sink. To improve the arsenal of complement therapies, tissue-targeting has emerged as a strategy to re-regulate complement in diseased tissue, while limiting systemic blockade. This approach, which is based on directing complement modulators to tissues through the recognition of tissue-fixed activated complement fragments, tissue-specific epitopes, or injury-associated neoepitopes, has the potential for enhanced potency and durability and reduced infection risk. In this review, we discuss the rationale for tissue-targeted complement therapies, the strategies taken to achieve local regulation, current state of preclinical and clinical stage tissue-targeted therapeutics, and potential future directions.

C3d-Targeted factor H inhibits tissue complement in disease models and reduces glomerular injury without affecting circulating complement

Complement-mediated diseases can be treated using systemic inhibitors. However, complement components are abundant in circulation, affecting systemic inhibitors' exposure and efficacy. Furthermore, because of complement's essential role in immunity, systemic treatments raise infection risk in patients. To address these challenges, we developed antibody fusion proteins combining the alternative-pathway complement inhibitor factor H (fH1-5) with an anti-C3d monoclonal antibody (C3d-mAb-2fH). Because C3d is deposited at sites of complement activity, this molecule localizes to tissue complement while minimizing circulating complement engagement. These fusion proteins bind to deposited complement in diseased human skin sections and localize to activated complement in a primate skin injury model. We further explored the pharmacology of C3d-mAb-2fH proteins in rodent models with robust tissue complement activation. Doses of C3d-mAb-2fH >1 mg/kg achieved >75% tissue complement inhibition in mouse and rat injury models while avoiding circulating complement blockade. Glomerular-specific complement inhibition reduced proteinuria and preserved podocyte foot-process architecture in rat membranous nephropathy, indicating disease-modifying efficacy. These data indicate that targeting local tissue complement results in durable and efficacious complement blockade in skin and kidney while avoiding systemic inhibition, suggesting broad applicability of this approach in treating a range of complement-mediated diseases.

Abstract 4090: Defeating primary checkpoint resistance: SRTβ1-Ab3 is a first-in-class, fully human antibody that renders resistant tumors sensitive to anti-PD-1

Despite the clinical breakthroughs achieved by checkpoint blockade therapy (CBT), a majority of patients treated with PD-(L)1 inhibitors fail to respond due to primary or acquired resistance. TGFβ signaling has recently been implicated as a mechanism of primary resistance to CBT, very likely via mechanisms that include immune exclusion. However, therapeutic targeting of the TGFβ pathway has been hindered by dose-limiting cardiotoxicities, most likely due to inhibition of signaling from multiple TGFβ isoforms. Upon secretion, TGFβ growth factor is held dormant in a latent complex with its non-covalently associated prodomain. TGFβ activation is triggered by extracellular events, such as integrin binding or proteolytic cleavage, that release the growth factor from this latent complex. We have demonstrated that isoform-specific inhibition of TGFβ activation can be achieved by targeting the prodomain to stabilize the latent TGFβ complex. We recently identified TGFβ1 as the predominant isoform in many human cancers, especially those for which CBT is approved for therapeutic intervention. SRTβ1-Ab3 is a fully-human antibody against latent TGFβ1 that inhibits its activation without binding or inhibiting latent TGFβ2, latent TGFβ3, or the active TGFβ1 growth factor. In syngeneic tumor models of primary CBT resistance, pharmacologic blockade of TGFβ1 activation with SRTβ1-Ab3 is sufficient to sensitize TGFβ1-predominant tumors to PD-1 inhibition. Mechanistically, combination treatment with anti-PD-1/SRTβ1-Ab3 overcomes immune exclusion in these models, induces CD8+ T cell infiltration into the tumors, and results in a reduction of myeloid immunosuppressive cells. In contrast, monotherapy with either anti-PD-1 or SRTβ1-Ab3 alone has only modest effects on these cell populations. Gene expression profiling of single vs. combination treated tumor samples provides a molecular view of effects on signaling pathways, cell populations, and activation status of these cells. These data demonstrate the efficacy of TGFβ1-specific inhibition in combination with anti-PD-1 in multiple mouse models of primary checkpoint resistance. Taken together, these synergistic effects at the tumor, cellular, and molecular levels, the preclinical safety profile, and the pharmacokinetic properties of SRTβ1-Ab3 establish a strong rationale for advancing the development of SRTβ1-Ab3 toward clinical application in cancer immunotherapy.

Citation Format: Thomas Schürpf, Constance J. Martin, Christopher Littlefield, Christopher Chapron, Stefan Wawersik, Ashish Kalra, Allison Simpson, Francis Danehy, Christopher Boston, Anastasia Nikiforov, Susan Lin, Justin Jackson, Gregory J. Carven, Alan Buckler, Abhishek Datta. Defeating primary checkpoint resistance: SRTβ1-Ab3 is a first-in-class, fully human antibody that renders resistant tumors sensitive to anti-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 4090.