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456 100% complete response to PEEL-224 in combination with PD-1 blockade in a syngeneic mouse model of colon cancer
  1. Aleah F Caulin1,
  2. Sophia Howard1,
  3. Trent Fowler1,
  4. Ivan Alferiev2,
  5. Michael Chorny2,
  6. Garrett Brodeur2,
  7. Debra Thoma Vallner1,
  8. Robert G Maki1,3 and
  9. Joshua D Schiffman1,4
  1. 1Peel Therapeutics, Salt Lake City, UT, USA
  2. 2Children’s Hospital of Philadelphia and the University of Pennsylvania/Perelman School of Medicine, Philadelphia, PA, USA
  3. 3Memorial Sloan Kettering Cancer Center, New York, NY, USA
  4. 4Huntsman Cancer Institute, University of Utah, Salt Lake City, UT, USA
  • Journal for ImmunoTherapy of Cancer (JITC) preprint. The copyright holder for this preprint are the authors/funders, who have granted JITC permission to display the preprint. All rights reserved. No reuse allowed without permission.


Background While immunotherapy has immense clinical value for selected patients, novel therapeutic combinations can increase the response rate and improve overall patient outcomes. Topoisomerase 1 (TOP1) inhibitors demonstrate increased cancer immunogenicity and may increase tumor response when combined with PD-1 blockade.1–3 PEEL-224 is an engineered TOP1 inhibitor comprised of a 4-arm polyethylene glycol (PEG) scaffold conjugated to four camptothecin-derived bioactive molecules. PEEL-224’s payload has been designed to overcome efflux-mediated chemoresistance and demonstrates increased bioavailability and tumor accumulation.4 PEEL-224 shows robust antitumor activity as monotherapy in nonclinical studies with multiple solid tumors. For the current study, we combined PEEL-224 with PD-1 blockade in a syngeneic mouse model of colon cancer.

Methods C57BL/6 female mice were injected subcutaneously with MC38 cells in Matrigel at 6–8 weeks old. Mice (N=10/group) received treatment of vehicle, PEEL-224, anti-mPD-1 (InVivoPlus Clone RMP1–14), or combined treatment. Vehicle and PEEL-224 were administered intravenously once weekly x 4 doses and anti-mPD-1 was administered intraperitoneally once every 3 days for 4 doses. Tumor volumes were measured 2–3 times/week for 6 weeks. Individual tumor response was assessed via the following criteria: Partial Response (PR), 30–99% decrease in tumor volume from Day 1 for ≥ 3 consecutive measurements; Complete Response (CR), 100% decrease in tumor volume from Day 1; No Response (NR), tumors did not meet PR or CR criteria. Statistical analyses to compare tumor volumes used one-way ANOVA with Tukey’s post-hoc testing for multiple comparisons. Response rates were compared with Fisher’s exact test.

Results PEEL-224 and anti-mPD-1 demonstrated robust antitumor activity as monotherapy and combined versus vehicle (p-value < 0.0001). PEEL-224 combined with anti-mPD-1 resulted in significantly more CRs than anti-mPD-1 alone (100% vs 30% CR; p-value < 0.05; table 1). In the combined treatment cohort, 3 tumors shrank to < 22 mm3 and 7 tumors completely regressed and could not be found at necropsy. While all tumors treated with combination therapy were considered PR or CR based on caliper measurements, pathology review revealed that the 3 remaining masses were devoid of any neoplastic cells. As a result, the combination of PEEL-224 and anti-mPD-1 was considered to have 100% CR rate.

Conclusions Our results indicate that combined PEEL-224 and PD-1 inhibition may improve antitumor efficacy over each monotherapy. PEEL-224 is currently in early-phase clinical trials to determine the single-agent recommended phase 2 dose (NCT05329103). This data supports future testing of PEEL-224 in combination with PD-1 blockade for solid tumors.


  1. Huang KC, et al. Immunogenic Cell Death by the Novel Topoisomerase I Inhibitor TLC388 Enhances the Therapeutic Efficacy of Radiotherapy. Cancers (Basel), 2021. 13(6).

  2. Lee JM, et al. Inhibition of topoisomerase I shapes antitumor immunity through the induction of monocyte-derived dendritic cells. Cancer Lett, 2021. 520: 38–47.

  3. McKenzie JA, et al. The Effect of Topoisomerase I Inhibitors on the Efficacy of T-Cell-Based Cancer Immunotherapy. J Natl Cancer Inst, 2018. 110(7): 777–786.

  4. Nguyen F, et al. Structural Optimization and Enhanced Prodrug-Mediated Delivery Overcomes Camptothecin Resistance in High-Risk Solid Tumors. Cancer Research, 2020: canres.1344.2020.

Ethics Approval This study was reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) at TD2 (approval #22106).

Abstract 456 Table 1

Tumor response summary based on tumor volume and histopathology

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