SUMMARY

• Talquetamab is a humanized immunoglobulin G4 (IgG4) bispecific DuoBody^® antibody with an IgG4-poly acrylic acid (PAA) scaffold that binds to both the Gprotein-coupled receptor class C group 5 member D (GPRC5D) on target cells and the epsilon (ε) chain of cluster of differentiation (CD)3 on T cells.¹
• Preclinical studies demonstrated that talquetamab prevents tumor growth and regresses established tumors by binding to both CD3 on T cells and GPRC5D-expressing cells, resulting in a cytotoxic response against GPRC5D-expressing cells.^1,3
  • Talquetamab effectively induced T-cell-dependent lysis of GPRC5D-expressing multiple myeloma (MM) cell lines in vitro and primary MM cells in patient samples, ex vivo.^1,3 This cytotoxic activity was lacking in GPRC5D-negative cells or MM cells treated with negative control antibodies.¹
  • Treatment with talquetamab resulted in increased activated T cells in MM tumor samples; led to the secretion of interferon (IFN)-γ, interleukin (IL)-2, IL-8, IL-10, and tumor necrosis factor (TNF)-α, which was consistent with Tcell activation; and Tcell proliferation in a dose-dependent manner.¹
  • In bone marrow samples obtained from patients with newly diagnosed MM (NDMM) and relapsed/refractory MM (RRMM), talquetamab effectively induced MM cell lysis, with a modest increase in natural killer (NK) cells and a small decrease in T-cell counts.⁴
  • In human MM xenograft models, talquetamab exhibited significant antitumor activity, with concomitant Tcell activation, compared with phosphate-buffered saline (PBS) and negative control antibodies.¹

BACKGROUND - OVERVIEW

Role of GPRC5D in MM Therapy

• GPRC5D is a 7-segment transmembrane orphan receptor protein that belongs to the family of G-protein-coupled receptors.^2,5 It is predominantly expressed in cells with a plasma-cell phenotype, with higher expression levels on the surface of malignant plasma cells in patients with MM, thereby making it a potential immunotherapeutic target.^2,4,6-8 GPRC5D can be expressed as a protein, as confirmed by immunohistochemistry (IHC), or messenger ribonucleic acid (mRNA), as confirmed by in situ hybridization (ISH).^9,10 It is found to be expressed as a protein in the lungs^2,9; as mRNA in the brain stem^2,10, in the keratogenous zone of nails¹¹, the spleen¹, and lymph nodes¹; and both as a protein and mRNA in skin tissues containing keratin-expressing hair follicles^2,10,11, the hard keratinized tissues of the tongue (ie, filiform papillae)^2,10,11, and resident or interstitial plasma cells within the salivary glands and tonsils.¹⁰ GPRC5D mRNA or protein has not been detected in the cerebral cortex, basal ganglia, midbrain, or cerebellum by ISH or IHC.² GPRC5D is regulated independently of Bcell maturation antigen (BCMA), which is consistent with heterogeneous expression.^1,9
• GPRC5D is highly expressed in MM cells compared to other hematologic malignancies, and in bone marrow cells of patients with smoldering MM. This increased expression has been associated with a high number of genetic abnormalities and high-risk disease, potentially due to elevated levels of GPRC5D expression in the malignant plasma cells in bone marrow samples. Increased expression of GPRC5D mRNA in the bone marrow has also been associated with poor prognosis and disease outcomes, including shorter progression-free survival and overall survival. As a causative relationship between GPRC5D and malignant plasma cell transformation or high-risk disease has not been demonstrated, these observations may be due to an overall increase in tumor burden and a higher proportion of high-risk disease features.²
• GPRC5D expression is unaffected by the loss of BCMA, which has been associated with disease relapse following treatment with BCMA-targeting therapies.² A summary of ongoing clinical trials of Talquetamab in patients with RRMM is detailed in Table: Summary of Ongoing Clinical Trials of Talquetamab.
• In an in vitro study conducted using Chinese hamster ovary cells engineered to express human GPRC5D cocultured with CD3+ T cells, GPRC5D/CD3 bispecific antibodies ligated human CD3⁺ T cells and GPRC5D-expressing cells, leading to the activation of human T cells in a manner dependent on their affinity to GPRC5D. Additionally, GPRC5D/CD3 bispecific antibodies induced cytotoxicity against GPRC5D-expressing MM cell lines with unstimulated human peripheral blood mononuclear cells (MNCs). In an in vivo study conducted using mouse xenograft tumors, GPRC5D/CD3 bispecific antibodies exhibited potent antitumor activity against GPRC5Dexpressing MM cell lines through the activation of T cells.⁶

Mechanism of Action

Talquetamab is a humanized IgG4 bispecific DuoBody^® antibody with an IgG4PAA scaffold that binds to both GPRC5D on target cells and the ε chain of CD3 on T cells.¹

Preclinical studies demonstrated that talquetamab prevents tumor growth and regresses established tumors by binding to both CD3 on T cells and GPRC5D-expressing cells, resulting in activation of T cells and subsequent lysis of GPRC5D-expressing cell through secretion of perforin and granzymes.^1,3,4 Data from preclinical studies that evaluated the mechanism of action of talquetamab are summarized below.

Preclinical Activity of Talquetamab in MM Cell Lines

In a preclinical study, talquetamab effectively lysed GPRC5D-expressing MM cell lines and primary MM cells in samples derived from both newly diagnosed and heavily pretreated patients, including those who were daratumumab refractory, with concomitant Tcell activation. Moderate correlations were found between maximum lysing by talquetamab and GPRC5D expression on MM cells (Spearman correlation coefficient [r]=0.45; P=0.006) and the percentage of regulatory T cells (r=-0.35; P=0.042).³

T-Cell-Mediated Cytotoxicity Induced by Talquetamab

In an in vitro study, GPRC5D-expressing MM cell lines with varying GPRC5D expression levels were incubated with purified healthy human T cells at a 5:1 effector/target ratio in the presence of talquetamab. Talquetamab promoted efficient Tcell-mediated cytotoxicity in all GPRC5D-expressing cells but had no effect on GPRC5Dnegative cell lines. The negative control bispecific antibodies had no cytotoxic effect. Similarly, talquetamab induced Tcell activation when cultured with GPRC5Dexpressing cells, as evidenced by increased CD25 expression on T cells. However, no significant Tcell activation was noted with GPRC5D-negative cells or negative control antibodies. Treatment with talquetamab led to the secretion of IFN-γ, IL-2, IL-8, IL10, and TNF-α, which was consistent with Tcell activation, and resulted in increased Tcell proliferation in a dose-dependent manner.¹

T-Cell-Mediated Cytotoxicity of GPRC5D-Expressing Cells in Healthy Human Whole Blood and Plasma Cells of Patients With MM

The cytotoxic activity of talquetamab was evaluated in a more physiologic environment using 2 different approaches.¹

In the first approach, when talquetamab was added to healthy human whole blood in the presence of GPRC5D-expressing H929 MM cells, it actively lysed GPRC5D-expressing cells in all 6 donor samples; this activity correlated with Tcell activation. When talquetamab was added to whole blood without GPRC5D-expressing H929 MM cells, it did not induce T-cell activation as a result of low levels or a lack of circulating plasma cells. Binding studies by fluorescence-activated cell sorting (FACS) demonstrated that talquetamab only bound to T cells and NK T cells in whole blood due to the presence of CD3ε on these cells, and these studies also supported the observed lack of cytotoxicity in the absence of H929 MM cells.¹

In the second approach, talquetamab was added to MNCs from freshly thawed bone marrow of patients with MM supplemented with exogenous T cells from healthy donors. Talquetamab could bind and deplete primary CD138⁺ cells in 4 MM samples with varying GPRC5D expression levels in a concentration-dependent manner, with concomitant Tcell activation. Low or no cytotoxicity or Tcell activation was observed with negative control antibodies.¹

T-Cell-Mediated Cytotoxicity of GPRC5D-Expressing MM and Primary MM Cells Obtained From Patients With NDMM and RRMM

In an ex vivo study, the activity of talquetamab was analyzed in 4 MM cell lines (3 GPRC5D positive and 1 GPRC5D negative) incubated with serial concentrations of talquetamab for 48 hours in the presence of healthy donorderived peripheral blood MNCs. Talquetamab did not affect the viability of GPRC5D-negative cells but was associated with dose-dependent lysis of the 3 GPRC5D-positive cell lines, with nearcomplete elimination of MM cells starting at a dose of 0.16 µg/mL of talquetamab. Administration of talquetamab led to dose-dependent activation and degranulation of both CD4⁺ and CD8⁺ T cells, as confirmed by the increased cell surface expression of CD25 and CD107a, respectively, which was consistent with a Tcell-dependent mode of action. Additionally, Tcell activation was associated with increased secretion of IFN-γ, TNF-α, IL-2, IL-4, IL-6, IL-10, IL-17A, and granzyme B in 2 MM cell lines.⁴

In bone marrow samples obtained from 45 patients with NDMM and RRMM, talquetamab effectively induced MM cell lysis after 48 hours of incubation, with a modest increase in NK cells and a small decrease in Tcell counts. The negative control antibodies demonstrated significantly lower anti-MM activity in these patient samples, confirming the requirement for cross-linking of target cells and T cells and ruling out the direct effect of GPRC5D targeting.⁴

Antitumor Activity of Talquetamab Associated With Tcell Activation in Human MM Xenograft Models

In an in vivo study, the antitumor activity of talquetamab was assessed in 2 tumor models of MM: a prophylactic H929 model to assess tumor formation prevention and an MM.1S model to assess regression of established tumors.¹

In the prophylactic H929 model, talquetamab completely prevented tumor formation at both 1 µg and 10 µg/mouse doses (P≤0.05). In the MM.1S model, statistically significant antitumor activity was observed at 10 µg and 50 µg/mouse doses, with 10 of 10 mice showing complete responses (tumor regression) in each group. The 1 µg/mouse dose significantly inhibited tumor growth by 65% (P≤0.05) compared with phosphate-buffered saline (PBS)-treated control animals. Negative control antibodies did not suppress tumor growth.¹

The antitumor activity of talquetamab was associated with a concomitant increase in activated T cells (CD25⁺, CD4⁺, and CD8⁺ T cells) as observed in the tumor samples at day 16 since the first dose. PBS and negative control antibodies did not reduce MM.1S tumor cells or activate T cells in tumor or circulating blood.¹

Immunohistochemical staining of tumor fragments showed a significant increase in Tcell infiltration in tumor samples from animals treated with talquetamab. A qualitative increase in CD3⁺, CD8⁺, and CD4⁺ T cells was noted in the talquetamab-treated tumor samples compared with the PBS and negative control antibody-treated tumor samples at day 19.¹

Literature search

A literature search of MEDLINE^®, Embase^®, BIOSIS Previews^®, and Derwent Drug Filedatabases (and/or other resources, including internal/external databases) pertaining to this topic was conducted on 05 February 2025.