Study of HPN424 in Patients With Advanced Prostate Cancer

Key Inclusion Criteria:

1. Male patients ≥18 years of age

2. Histologically or cytologically confirmed adenocarcinoma of the prostate

3. Progressive metastatic castrate-resistant prostate cancer (mCRPC): a) serum
testosterone levels less than 50 ng/dL (or ≤0.50 ng/mL or 1.73 nmol/L) within 28 days
prior to start of study drug, b) radiographic evidence of metastatic disease and c)
disease progression on the prior systemic regimen,

4. Must have received at least 2 prior systemic therapies approved for mCRPC

5. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

6. Adequate bone marrow function, including:

7. Able to read, understand and provide written informed consent

Key Exclusion Criteria:

1. Untreated brain metastases. Participants must have completed treatment for brain
metastasis, and be neurologically stable off steroids, for at least 4 weeks prior to
first dose of study drug.

2. Untreated spinal cord compression. Participants must be neurologically stable off
steroids for at least 4 weeks prior to first dose of study drug.

3. Concurrent treatment with anti-tumor necrosis factor (TNF) alpha therapies, systemic
corticosteroids (prednisone dose >10 mg per day or equivalent), or other immune
suppressive drugs within the 2 weeks prior to Screening

4. History of or known or suspected autoimmune disease (exception(s): patients with
vitiligo, resolved childhood atopic dermatitis, hypothyroidism, or hyperthyroidism
that is clinically euthyroid at Screening are allowed)

5. History of clinically significant cardiovascular disease

6. Active or chronic hepatitis B or hepatitis C as demonstrated by hepatitis B surface
antigen (HBsAg) positivity and/or anti-hepatitis C virus (HCV) positivity,
respectively, or known history of human immunodeficiency virus (HIV) seropositive
status

7. Clinically active or chronic liver disease,

8. Second primary malignancy that has not been in remission for greater than 3 years.