Nonclinical Development of SRK-181: An Anti-Latent TGFβ1 Monoclonal Antibody for the Treatment of Locally Advanced or Metastatic Solid Tumors

Abstract

Checkpoint inhibitors offer a promising immunotherapy strategy for cancer treatment; however, due to primary or acquired resistance, many patients do not achieve lasting clinical responses. Recently, the transforming growth factor-β (TGFβ) signaling pathway has been identified as a potential target to overcome primary resistance, although the nonselective inhibition of multiple TGFβ isoforms has led to dose-limiting cardiotoxicities. SRK-181 is a high-affinity, fully human antibody that selectively binds to latent TGFβ1 and inhibits its activation. To support SRK-181 clinical development, we present here a comprehensive preclinical assessment of its pharmacology, pharmacokinetics, and safety across multiple species. In vitro studies showed that SRK-181 has no effect on human platelet function and does not induce cytokine release in human peripheral blood. Four-week toxicology studies with SRK-181 showed that weekly intravenous administration achieved sustained serum exposure and was well tolerated in rats and monkeys, with no treatment-related adverse findings. The no-observed-adverse-effect levels levels were 200 mg/kg in rats and 300 mg/kg in monkeys, the highest doses tested, and provide a nonclinical safety factor of up to 813-fold (based on C_max) above the phase 1 starting dose of 80 mg every 3 weeks. In summary, the nonclinical pharmacology, pharmacokinetic, and toxicology data demonstrate that SRK-181 is a selective inhibitor of latent TGFβ1 that does not produce the nonclinical toxicities associated with nonselective TGFβ inhibition. These data support the initiation and safe conduct of a phase 1 trial with SRK-181 in patients with advanced cancer.

Keywords: SRK-181; TGFβ inhibitors; cancer immunotherapy; latent TGFβ1.

Figure 1.

SRK-181 inhibits activation of latent transforming growth factor-β (TGFβ1) from human, rat, and cynomolgus monkey. (A) LN229 human glioblastoma cells were used to overexpress human (circles), rat (squares), or cynomolgus (triangles) latent TGFβ1 in the extracellular matrix. Latent TGFβ1 activation was measured with CAGA12 reporter cells and expressed as percent. One hundred percent activity was determined in the presence of vehicle control. Data shown are mean ± standard deviation from 2 independent triplicate experiments. Curves are best-fits to a dose–response inhibition model. (B) SRK-181 IC50 values for inhibition of latent TGFβ1 activation, by species.

Figure 2.

SRK-181 has no effect on human platelet aggregation, activation, and binding. (A) Platelet aggregation with ADP agonist was not altered by SRK-181 as compared to vehicle control. (B) Platelet activation with ADP agonist was not affected by increasing concentration of SRK-181, compared with the vehicle control. (C) SRK-181 binding to unactivated or activated platelets was low and did not increase with higher concentrations, as compared to vehicle control. Data shown in panel (A) are the mean of 4 donors ± standard deviation, while data shown in panels (B) and (C) are replicate means from one donor ± standard deviation and are representative of 4 donors.

Figure 3.

SRK-181 does not induce cytokine production in healthy human peripheral blood mononuclear cells. Cytokine production in response to SRK-181 (triangles), positive anti-CD3/anti-CD28 control (circles), and vehicle IgG control (squares) was determined in a plate-based cytokine release assay. Summarized results are across 5 to 8 healthy human donors. In some instances, for interleukin (IL)-1β and IL-6, the positive control values were above the upper limit of quantitation (ULOQ) and thus are indicated along the dashed line depicted on the graph. Analytes measured below the limit of quantitation (typically approximately 1-5 pg/mL) were entered as “1” for the purposes of graphing.

Figure 4.

Serum concentration time profile for SRK-181 following a single dose. A single intravenous (IV) bolus dose of SRK-181 (or SRK-181-mIgG1, for mice) was administered at (A) 0.3, 3, 10, and 30 mg/kg in female C57BL/6 mice, (B) 0.3, 1, 3, 10, and 30 mg/kg in female SD rats and (C) 1, 3, 10, and 30 mg/kg in female cynomolgus monkeys. In all studies, Cmax increased approximately dose proportionally while area under the concentration time curve (AUC) increased less than dose proportionally, suggesting TMDD. Data shown are mean ± standard deviation. Dose symbols: 0.3 (circles), 1 (downward triangles), 3 (squares), 10 (triangles), and 30 (diamonds) mg/kg.

Figure 5.

Serum concentration time profile of SRK-181 following multiple doses. (A) Male and female SD rats were administered an intravenous (IV) bolus dose of SRK-181 at 30, 100, or 200 mg/kg on days 1, 8, 15, 22, and 29. C_max, area under the concentration time curve (AUC)0-168, and AUC0-696 (on day 29) increased approximately dose proportionally from 30 to 300 mg/kg and there were minimal-to-no differences between sexes. There were no confirmed Antidrug antibody (ADA) positive samples in the 100 or 200 mg/kg SRK-181 treatment groups nor any decrease in exposure consistent with an immune response. (B) Male and female cynomolgus monkeys were administered an IV bolus dose of SRK-181 at 30, 100, or 300 mg/kg on days 1, 8, 15, 22, and 29. Cmax and AUC0-168 increased approximately dose proportionally from 30 to 300 mg/kg, and there were minimal-to-no differences between sexes. There were no confirmed ADA positive samples in any SRK-181 treatment group nor any decrease in exposure consistent with an immune response. Data shown are mean ± standard deviation. Dose symbols: 30 (diamonds), 100 (triangles), 200 (squares), and 300 (circles) mg/kg.