This was a multi-center, open-label, Phase 1, first-in-human (FIH), dose-escalation, and dose expansion study to evaluate the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and efficacy of SRK-181 administered alone and in combination with anti-PD-(L)1 therapy in adult patients with locally advanced or metastatic solid tumors. The study was divided into 3 treatment parts (Part A1, Part A2, and Part B) and a Long-Term Extension Phase (LTEP).

Key Inclusion Criteria:

• Patient has a histologically documented solid tumor that is metastatic or locally advanced, for which SoC therapy does not exist, has failed in the patient, or is not tolerated by the patient, or for which the patient has been assessed by the Investigator as not being a suitable candidate or otherwise ineligible for the SoC therapy.

• For Part A2:

  o Patient must have a history of anti-PD-(L)1 antibody nonresponse presenting (based upon the Investigator's assessment) either as progressive disease or stable disease (e.g., not improving, but also not worsening, clinically or radiographically) after at least 3 cycles of treatment with the most recent anti-PD-(L)1 antibody therapy (alone or in combination with chemotherapy) approved for that tumor type. (Note: if the duration of prior anti-PD-1 therapy is shorter than 3 cycles and the reason for discontinuation is progressive disease, the progression should be associated with clinical deterioration.)

• For Part B Cohort NSCLC, UC, MEL and ccRCC:

  • Patient must be diagnosed with one of the following disease-specific solid tumors of NSCLC, UC, or MEL, and must have a history of primary nonresponse to anti-PD-1 therapy (alone or in combination with other therapy), presenting the best response (based upon the Investigator's assessment) either as progressive disease or stable disease (e.g., not improving, but also not worsening, clinically or radiographically) after at least 3 cycles of treatment.
  • For Cohort NSCLC, patients who have genomic tumor aberrations for which a targeted therapy is available (e.g., anaplastic lymphoma kinase, EGFR) must have progressed on an approved therapy for these aberrations or did not tolerate an approved therapy for these aberrations, or were not considered suitable candidates/ were otherwise ineligible for an approved therapy for these aberrations.
  • For Cohort ccRCC, patients must have a histologically confirmed diagnosis of RCC with a predominant clear cell component and must have received at least 1 prior line of anti-PD-1 treatment (alone or in combination with other therapy) and have had disease progression clinically or radiographically on the most recent anti-PD-1 treatment
  • Up to 3 lines of treatment are allowed between the last dose of anti-PD-1 and enrollment.

• For Part B Cohort HNSCC:

  • Patients must have a histologically confirmed diagnosis of recurrent or metastatic HNSCC that is non-amendable to curative therapy (e.g., radiation or surgery).
  • The primary tumor location must be the oropharynx, oral cavity, hypopharynx, or larynx. Primary tumor site of nasopharynx (any histology) or unknown primary tumor are not eligible.
  • Patients must have received one prior line of anti-PD-1 treatment (alone or in combination with other therapy) and have had disease progression clinically or radiographically on the anti-PD-1 treatment.
  • Up to one line of treatment are allowed between the last dose of anti-PD-1 and enrollment.
  • For patients with primary oropharyngeal cancer, patients must have results from testing of human papillomavirus (HPV) or P16 status.

• For Part B Cohort Any Other (enrollment complete): Patient must be diagnosed with any other solid tumor type that is not NSCLC, UC, MEL, or ccRCC for which the patient has had a history of primary anti PD (L)1 antibody nonresponse, presenting the best response (based upon the Investigator's assessment) as progressive disease, after prior anti-PD-(L)1 antibody therapy (alone or in combination with other therapy) currently approved for that tumor indication

• Measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 as assessed at Screening.

• Patient must have an Eastern Cooperative Oncology Group performance status (PS) 0-1.

• Patient must have a predicted life expectancy of ≥ 3 months.

• Women of child-bearing potential (WOCBP) must have a negative urine or serum pregnancy test up to 24 hours prior to first dose of SRK-181.

• WOCBP and males with female partners of childbearing potential must agree to use adequate birth control throughout their participation and for 90 days following the last dose of SRK-181.

Key Exclusion Criteria:

• For Part A1 only:

  • Patient has had anti-PD-(L)1 antibody therapy ≤ 28 days prior to the first dose of SRK-181.
  • Patient is receiving concurrent anti-cancer treatment, including anti-PD-(L)1 antibody therapy, either approved or investigational, within 28 days prior to the first dose of SRK-181.

• For Part A2 and Part B only:

  • Patient is receiving concurrent anti-cancer treatment, with the exception of an anti-PD-(L)1 antibody therapy for Part A2 or Part B, either approved or investigational, within 28 days prior to the first dose of SRK-181.
  • Patient has received biologic therapy (except for anti-PD-(L)1 antibody therapy for Part A2 or Part B), <28 days prior to the first dose of SRK-181.
  • Patient has received systemic cytotoxic chemotherapy (except for in combination with anti-PD-(L)1 antibody therapy) <28 days prior to the first dose of SRK-181.
  • Patient has received targeted small molecule therapy within 5 half-lives of the compound prior to the first dose of SRK-181.
  • Patient has a history of intolerance or treatment discontinuation due to severe irAE or other adverse reaction from prior anti-PD-(L)1 antibody therapy.

• Patient has a hypersensitivity to anti-PD-(L)1 antibody therapy.

• Patient has the documented presence of neutralizing ADA to anti-PD-(L)1 antibody therapy.

• Patient has a diagnosis of immunodeficiency, either primary or acquired.

• Patient is symptomatic or has uncontrolled brain metastases, leptomeningeal disease, or spinal cord compression not definitively treated with surgery or radiation.

• Patient has current second malignancy at other sites (exceptions: adequately treated in situ carcinoma [e.g., cervical], non-MEL skin cancer, bilateral synchronous discordant breast cancer, or indolent prostate cancer under observation). A past history of other malignancies is allowed as long as patient has been free of recurrence for ≥ 2 years, or if the patient has been treated with curative intent within the past 2 years and, in the opinion of the Investigator, is unlikely to have a recurrence.

• Women who are pregnant or breastfeeding.