Nublar Analysis: in this blog entry, we provide a summary of the Nature publication, accompanied by our curated industry-specific examples to illustrate tangible progress in CAR immune cell tech.
Durability of disease control is achieved in only 50% of patients even with the most clinically advanced CAR-T products, i.e., autologous B cell-directed, which remain resource intensive and limited in scalability despite manufacturing innovations to reduce turnover time
Target Antigen Density
A major topic explored in the paper is the capacity of (CAR)-T cells to recognize and bind target cancer neoantigens / tumor associated antigens (TAAs)
The antigen binding sensitivity of TCR-CD3 complexes (i.e., T cell receptor signaling) is greater than that of CAR scFv constructs (monoclonal antibody-like domains), thus the latter require higher thresholds of antigen presentation
Increasing T cell fitness, which drives signal strength permits a lower threshold of antigen density for CARs
Antigen thresholds should be calibrated according to type of malignancy based on relative ubiquity (e.g., low threshold sufficient for B cells) and specificity of target molecules within disease tissues (e.g., high threshold required for TAAs)
Variability of Antigen Expression
Antigen modulation is a challenge for both hematological malignancies (e.g., variable BCMA expression) and solid tumors (e.g., heterogeneity of antigen presentation and density)
Multiple modulation mechanisms exist, including but not limited to epigenetic and post-transcriptional modification, genetic mutations, and antigen shedding
The paper discusses methods to counter target antigen downregulation, including small molecule induced upregulation of TAAs
Allogene's allogeneic BCMA CAR-T program (ALLO-715) for multiple myeloma implements the proprietary gamma-secretase inhibitor from SpringWorks Therapeutics – nirogacestat – to elicit this effect in patients with low BCMA expression in the Phase I UNIVERSAL trial (NCT04093596)
This approach was validated in a preclinical publication from AACR's Cancer Reserach Communications
T Cell Dysfunction
T cell dysfunction, often characterized as T cell exhaustion, can occur due to extrinsic factors (e.g., high tumor burden) but also intrinsic factors (hyperactive and/or tonic co-stimulation) Overcoming T cell dysfunction is key to persistence, but authors note that in certain malignancies, increasing persistence may be deleterious to long term remission, requiring further consolidation therapy
Regarding solid tumors, advancements in co-stim domains and potency enhancements to combat TME immunosuppression are being evaluated, such as:
Novel co-stim constructs: Ginkgo Bioworks’ 2nd generation intracellular domain (ICD) library
Membrane bound / co-expressed IL-15: Precigen’s PRGN-3005 UltraCAR-T
IL-15 armoring: Adicet’s ADI-002 γδ CAR-T
IL-12 tumor-specific secretion: Cellectis’ "multi-armored" MUC-1 CAR-T
Inducible activation of CD40 (adaptive) and MyD88 (innate) immune signaling: Bellicum’s BPX-601; UTC Therapeutics’ CD40L expressing MSLN-CAR-T
Intratumoral dendritic cell driven epitope spreading to combat antigen heterogeneity: University of Melbourne’s Flt3L-secreting CAR T
T Cell Trafficking
Inhibition of CAR-T trafficking is another major solid tumor challenge, though authors point to intracerebroventricular delivery inducing sustained complete response in glioblastoma (GBM)
Industry efforts in GBM and NBL (neuroblastoma) include:
IL-13Rα2 targeting: Mustang Bio’s MB-101; Senglang Bio’s SENL201
GD2 targeting CAR-T: Autolus’ AUTO6NG
Next-Generation CAR Platforms
The paper provides an excellent overview of next-generation CAR-T constructs and approaches, including multi-targeting CARs (bivalent / tandem CARs, multiple single-scFv CARs, etc.) either to enhance target binding or as ‘switches’ and ‘gates’ to modulate activity.
However, authors point to important associated technical challenges; a number of industry assets incorporate these approaches to enhance target specificity toward better efficacy and safety:
Source: Labanieh L and Mackall CL. CAR immune cells: design principles, resistance and the next generation. Nature. 2023 Feb; 614: 635-648.
Janssen and Legend Bio’s CARVYKTI (cilta-cel) for multiple myeloma boasts two BCMA binding domains
Senti Biosciences employs OR GATE and/or NOT GATE in SENTI-202 (AML, MDS, other heme) and SENTI-401 (CRC)
A2 Biotherapeutics' Tmod platform exploits the loss of heterozygosity (LOH) characteristic of tumor cells to increase precision using an AND GATE in solid tumors (A2B530 and A2B694)
Bellicum’s GoCAR-T platform has developed small molecule inducible switches to fine-tune CAR activation and signaling (BPX-601 and BPX-603) in solid tumors
A Dana-Farber- and MGH-led effort utilized lenalidomide to drive ON/OFF switches in a CAR-T product
Calibr and AbbVie’s universal CAR-T – CLBR001 – is only directed toward CD19-expressing cells in the presence of the SWI019 antibody, conferring a switch function
High Affinity Binding
Combining the strong stimulatory signal of CARs and the high antigen sensitivity of the TCR is a compelling approach discussed in the paper, including HLA-independent T cells (HIT) engineered to recognize low affinity targets.
Mnemo is advancing HIT technology, and has published preclinical work on their process in Nature
TCR2 Therapeutics already has encouraging Phase I/II data (DCR = 81%) for gavo-cel, their lead ‘TRuC’ product, a MSLN-targeting HIT that retains αβ TCR signaling while modifying the γδ TCR domains
The result is an scFv antigen binding domain coupled to a potent CD3-ζ activation domain, retaining all activation and control elements of the TCR, while binding to extracellular TAAs
Immatics’ ACTallo platform has developed allogeneic γδ T cells (HLA independent) +/- CARs, thus targeting surface proteins as well as intracellular peptides presented by pHLA complexes and which comprise ~80% of solid tumor neoantigens
This is in addition to more advanced Phase I assets: TCR-T (ACTengine) and TCR bispecifics (TCER) targeting solid tumor pHLA targets PRAME and MAGEA4/8
Gadeta and Immunocore are collaborating on bispecific γδ TCR-based HLA-independent soluble proteins for solid tumors
GDT201’s innovative design replaces the αβ TCR of a T cell with a γδ TCR, inherently capable of selectively targeting stressed (cancer) cells, while retaining the proliferative potential of the native T cell
Although Adaptimmune’s most clinically advanced (pre-registrational) autologous solid tumor TCR-T afami-cel is not HLA-independent, the company is developing an allogeneic iPSC-derived MSLN targeting HIT cell product
NEWS ALERT On March 6, Adaptimmune ($ADAP) and TCR2 Therapeutics ($TCRR) merged to form a "pre-eminent cell therapy company for solid tumors." (All-stock transaction)
The newfound company is expected to have a cash runway extending until 2026, with key catalysts including:
Afami-cel BLA submission (synovial sarcoma): completion by mid-2023 based on the SPEARHEAD-1 trial
ADP-A2M4CD8 (next-gen afami-cel) 2023 milestones:
Full data readout from Phase I SURPASS trial (NCT04044859) monotherapy cohort: heavily pretreated solid tumors
Initiation of SURPASS trial pembrolizumab combination cohorts for 1L HNSCC and 2L bladder cancer
Initiation of potentially registrational Phase 2 SURPASS-3 trial in platinum-resistant ovarian cancer (NCT05601752); combo with nivolumab
Gavo-cel initial readout from Phase II trial (NCT03907852) by EOY 2023
Nublar takeaway: two of the most promising TCR-based cell therapy companies pooled expertise, resources, and platforms (SPEAR and TRuC) to form what we expect to be an industry leading effort to optimize target affinity, target breadth (particularly in solid tumor), and operational prowess in clinical development.
Notable solid tumor targets implicated: PRAME, MSLN, MAGE-A4, CD70
Afami-cel and gavo-cel will now fall under the same portfolio. TCR tech for solid tumor could move the needle significantly.
Intracellular Peptide-Targeting CARs
Perhaps the most innovative development highlighted by the authors is the engineering of a CAR capable of recognizing a low affinity peptide coding for an epigenetic driver (PHOX2B) of high-risk neuroblastoma, which is presented across multiple HLA allotypes
The peptide is not expressed in healthy tissues, and shows resistance to antigen loss due to immunotherapy; this engineering effort is described in Nature
Source: Yarmarkovich M et al. Cross-HLA targeting of intracellular oncoproteins with peptide-centric CARs. Nature. 2021 Nov; 599(7885):477-484.
Allogeneic CAR-T
Authors discuss the emergence of allogeneic T cell platforms, which have the potential to overcome attendant logistical challenges such as vein-to-vein time, manufacturing cost, and scalability, as well as technical challenges such as poor T cell fitness of patient-derived product.
The paper highlights methods to overcome donor-derived CAR-T obstacles – principally eliminating graft vs. host (GvH) immune response, but also deleterious attacks from the host immune system (T cells, NK cells, macrophages, etc.) toward the allogeneic cell therapy product.
Of relevance is Allogene’s market leading allogeneic CAR-T asset ALLO-501A, which utilizes TALEN nuclease gene editing to knockout the TRAC (native TCR) locus to mitigate GvH
Allogene has already begun enrolling patients in the pivotal Phase II ALPHA2 trial (NCT04416984) in 3L+ DLBCL, per recent Q4 2022 earnings.
Other manipulations to optimize allogeneic CAR-T persistence, immune evasion, and safety include deletions / knockdowns or knock-ins of donor cell MHC expression via editing tools such as CRISPR (CTX-120), Cas-CLOVER and piggybac (P-MUC1C-ALLO1), ARCUS (azer-cel), and shRNA (Ceylad’s TIM platform), etc.
Authors note recent hurdles with gene editing in cell therapy, including the potential for genome disruption and chromosomal abnormalities, e.g., Allogene’s now-resolved clinical hold (case study detailed in Molecular Therapy) and point to potential product differentiation based on editing platform-specific fidelity, magnified over large scale manipulations
Source: Sasu BJ et al. Detection of chromosomal alteration after infusion of gene-edited allogeneic CAR T cells. Mol Ther. 2023 Mar; 31(3): 676-685.
Emerging Tech and Future Outlook for CAR-T
The article concludes by previewing upcoming engineering techniques such as in vivo CAR-T, site-specific insertion of the CAR to knockout the PD-1 locus on T cells (simultaneously combating an immune checkpoint), and small molecule-driven homology directed repair to improve CRISPR efficiency, among other techniques.
Authors also point to the application of CAR-T in indications beyond oncology such as autoimmune disease such as:
An academic mouse study of anti-CD19 CAR-T for Lupus
CAR-T in the emerging field of #senolytics and #longevity, where accumulation of the senescence associated secretory phenotype (SASP) is an upstream driver of multiple age-related pathologies, and toward which CARs have been targeted
A Nature paper discusses the targeting of cells expressing uPAR, a marker of cellular #senescence
Other than the above examples, referenced in the paper, notable industry programs in autoimmune disease include:
Sangamo Therapeutics' allogeneic CAR-Treg program led by TX200, currently under investigation in the Phase I/II STEADFAST trial (NCT04817774) for the prevention of immune-mediated rejection in HLA mismatched kidney transplantation
Among preclinical program indications leveraging Sangamo's CAR-T technology is Crohn's Disease, characterized by the expression of IL-23R in the GI tract
Source: Sangamo Therapeutics company website
A major challenge in AAV gene therapy is overcoming the humoral and cellular immune responses activated by exposure to the vector in patients who have previously been exposed to the same or a cross-reactive AAV serotype, including upon re-dosing the same vector.
Selecta Biosciences has developed a complementary technology to their based ImmTOR platform, comprised of nanoparticle encased rapamycin that generates antigen-specific tolerance against the putative immunogenic AAV peptide
ImmTOR-IL is an engineered IL-2 that not only expands Tregs per their native function, but selectively expands Tregs specific to a desired AAV peptide
The combination of ImmTOR and ImmTOR-IL delivers "precision immune tolerance" against AAV based gene therapy
Despite Selecta's robust pipeline planning in gene therapy, the company plans to initiate IND-enabling studies for ImmTOR-IL in 2023 in an autoimmune indication
Source: Selecta Biosciences company website.
Conclusion
The outlook for CAR-T, as well as for alternate T cell approaches is increasingly promising, and has advanced by leaps and bounds as biological and clinical challenges have been systematically defined and studied.
Despite these advances, clinical proof of concept is limited, particularly for solid tumor CAR-T and for allogeneic products. Further contributing to the uncertainty surrounding CAR and T cell evaluation is, paradoxically, the abundance of highly-specialized platforms, assets, and combinations, each which promise to overcome a given challenge, but which may introduce unintended negative consequences, as well as 'unknown unknowns'. New approaches also include accompanying pre-conditioning; cell therapy players continue to tinker with single dose optimization, calibration of re-dosing regimens where possible and potentially beneficial, and related lymphodepletion regimen considerations.
If 2022 provided any lesson, it's that #Biotech cash runways can contract significantly and swiftly, warranting well thought-out platform plays and focused clinical programs. Through the looking glass, rapid CAR-T innovation presents #Pharma with collaborative BD opportunities to move into what will represent the next major sector in #immunotherapy. Ultimately, bringing effective but well-vetted therapies, including for solid tumors, will be critical to delivering curative potential for patients.
NK cell, engineered dendritic cells, and CAR-Macrophage modalities are also finding their footing as new Biotechs forge the afore-referenced collaborations with Big Pharma, and with established Pharma players such as Sanofi and Takeda launching robust NK programs.
Generating durable efficacy, minimizing toxicity (namely CRS and neurotoxicity), circumventing solid tumor hurdles, and offering (CAR)-T in earlier lines of therapy will be benchmarks to follow.
More to come...
Author
Rohan D Gidvani, PhD
Director, Nublar Consulting
rohan@nublarconsult.com