A Multiple-dose Study of ASP8374, an Immune Checkpoint Inhibitor, as a Single Agent and in Combination With Pembrolizumab in Subjects With Advanced Solid Tumors

This is a multi-center, multiple-dose, dose-escalation and expansion study of ASP8374 as a
single agent and in combination with pembrolizumab. After discontinuation of study drug
treatment, all participants will complete an end of treatment visit and safety follow-up
visits. Participants will be enrolled in respectively escalation cohorts or expansion
cohorts.

Escalation cohorts: Approximately 63 participants may be enrolled in the escalation cohorts
(approximately 33 participants for monotherapy and 30 participants for combination therapy).

Expansion cohorts: The total number of subjects in the expansion cohorts will depend on the
observed pharmacokinetic and antitumor activity. It is estimated that approximately 300
participants may be enrolled in the monotherapy and combination therapy expansion cohorts.

As the number of participants in the escalation cohorts and the expansion cohorts will depend
on the observed Dose Limiting Toxicity (DLT), pharmacokinetics and antitumor activity,
approximately 363 participants are expected to be enrolled.

Inclusion Criteria:

- Subject has locally-advanced (unresectable) or metastatic solid tumor malignancy (no
limit to the number of prior treatment regimens) that is confirmed by available
pathology records or current biopsy as well as:

- Subject in the escalation cohort has received all standard therapies (unless the
therapy is contraindicated or intolerable) felt to provide clinical benefit for
the subject's specific tumor type. OR

- Subject in an expansion cohort has received at least one standard therapy for the
subject's specific tumor type.

- For Korea and Italy only: Subject has locally-advanced (unresectable) or metastatic
solid tumor malignancy (no limit to the number of prior treatment regimens) that is
confirmed by available pathology records or current biopsy and has received all
standard therapies (unless the therapy is contraindicated or intolerable) felt to
provide clinical benefit for the subject's specific tumor type.

- Subject has an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1 or
2.

- Subject's last dose of prior antineoplastic therapy, including any immunotherapy, was
at least 21 days prior to initiation of study drug administration. A subject with
epidermal growth factor receptor (EGFR) or anaplastic lymphoma kinase (ALK)
mutation-positive NSCLC is allowed to remain on EGFR tyrosine kinase inhibitor (TKI)
therapy or ALK inhibitor until 4 days prior to the start of study drug administration.

- For Korea only: Subject's last dose of prior antineoplastic therapy, including any
immunotherapy, was at least 21 days prior to initiation of study drug administration.
For drugs with a half-life greater than or equal to 21 days, the investigator should
consider if this washout is sufficient. A subject with epidermal growth factor
receptor (EGFR) mutation-positive non-small cell lung cancer (NSCLC) is allowed to
remain on EGFR tyrosine kinase inhibitor (TKI) therapy until 7 days prior to the start
of study drug administration.

- Subject has completed any radiotherapy (including stereotactic radiosurgery) at least
2 weeks prior to study drug administration.

- Subject's adverse events (excluding alopecia) from prior therapy have improved to
grade 1 or baseline within 14 days prior to start of study treatment.

- Subject with metastatic castration resistant prostate cancer (mCRPC) (positive bone
scan and/or soft tissue disease documented by computed tomography (CT) / magnetic
resonance imaging (MRI)) meets both of the following:

- Subject has serum testosterone ≤ 50 ng/dL at screening.

- Subject has had an orchiectomy or plans to continue androgen deprivation therapy
(ADT) for the duration of study treatment.

- Subject has adequate organ function prior to start of study treatment as indicated by
the following laboratory values. If a subject has received a recent blood transfusion,
the laboratory tests must be obtained ≥ 4 weeks after any blood transfusion.

- Female subject is eligible to participate if she is not pregnant and at least 1 of the
following conditions applies:

- Not a woman of childbearing potential (WOCBP)

- WOCBP who agrees to follow the contraceptive guidance throughout the treatment
period and for at least 6 months after the final study drug administration.

- Female subject must agree not to breastfeed starting at screening and throughout the
study treatment, and for 6 months after the final study drug administration.

- Female subject must not donate ova starting at screening and throughout the study
treatment, and for 6 months after the final study drug administration.

- A male subject with female partner(s) of childbearing potential must agree to use
contraception as detailed during the treatment period and for at least 6 months after
the final study drug administration.

- Male subject must not donate sperm starting at screening and throughout the study
treatment, and for 6 months after the final study drug administration.

- Male subject with a pregnant or breastfeeding partner(s) must agree to remain
abstinent or use a condom for the duration of the pregnancy or time partner is
breastfeeding throughout the study treatment and for 6 months after the final study
drug administration.

- Subject agrees not to participate in another interventional study while receiving
study drug (subjects who are currently in the follow-up period of an interventional
clinical trial are allowed).

Additional Inclusion Criteria for Subjects in the Expansion Cohorts:

- Subject meets one of the following:

- Subject has the tumor type for which a confirmed response was observed in a
monotherapy or combination therapy dose escalation cohort; or

- For an expansion cohort opened due to achieving predicted efficacious exposure,
subject has squamous cell carcinoma of the head and neck (SCCHN); or

- For tumor specific expansion cohorts of ASP8374 with pembrolizumab, subject has
the applicable tumor type (e.g., non-small cell lung cancer (NSCLC), bladder
cancer, gastric cancer, metastatic castration resistant prostate cancer (MCRPC)
or colorectal cancer (CRC)).

- Subject has at least 1 measureable lesion per Response Evaluation Criteria in Solid
Tumors (RECIST) 1.1. The measureable lesion must be outside the field of radiation if
subject had prior radiotherapy. Subjects with mCRPC who do not have measurable lesions
must have at least one of the following:

- Progression with 2 or more new bone lesions; or

- Prostate-specific antigen (PSA) progression (defined as a minimum of three rising
PSA levels with an interval of ≥ 1 week between each determination) within 6
weeks prior to study drug administration and a PSA value at the screening visit ≥
2 ng/mL.

- Subject consents to provide an available tumor specimen in a tissue block or unstained
serial slides obtained within 56 days prior to first dose of study treatment, or
subject is an appropriate candidate for tumor biopsy and is amenable to undergoing a
tumor biopsy (core needle biopsy or excision) during the screening period.This does
not apply to subjects with mCRPC without measurable disease.

- Subject in any expansion cohort, is an appropriate candidate for tumor biopsy and
consents to undergoing a tumor biopsy (core needle biopsy or excision) during the
treatment period as indicated in the Schedule of Assessments.

Exclusion:

- Subject weighs < 45 kg at screening.

- Subject has received investigational therapy (other than an investigational epidermal
growth factor receptor tyrosine kinase inhibitor (EGFR TKI) in a subject with EGFR
activating mutations or ALK inhibitor in a subject with an ALK mutation) within 21
days prior to start of study drug.

- Subject requires or has received systemic steroid therapy or any other
immunosuppressive therapy within 14 days prior to study drug administration. Subjects
using a physiologic replacement dose of hydrocortisone or its equivalent (defined as
up to 30 mg per day of hydrocortisone up to 10 mg per day of prednisone) are allowed.

- Subject has symptomatic central nervous system (CNS) metastases or subject has
evidence of unstable CNS metastases even if asymptomatic (e.g., progression on scans).
Subjects with previously treated CNS metastases are eligible, if subject is clinically
stable and have no evidence of CNS progression by imaging for at least 4 weeks prior
to start of study treatment and are not requiring immunosuppressive doses of systemic
steroids (> 30 mg per day of hydrocortisone or > 10 mg per day of prednisone or
equivalent) for longer than 2 weeks.

- Subject has an active autoimmune disease. Subjects with type 1 diabetes mellitus,
endocrinopathies stably maintained on appropriate replacement therapy, or skin
disorders (e.g., vitiligo, psoriasis, or alopecia) not requiring systemic treatment
are allowed.

- Subject was discontinued from prior immunomodulatory therapy due to a grade ≥ 3
toxicity that was mechanistically related (e.g., immune related) to the agent.

- Subject has known history of serious hypersensitivity reaction to a known ingredient
of ASP8374 or pembrolizumab or severe hypersensitivity reaction to treatment with
another monoclonal antibody.

- Subject has a known history of Human Immunodeficiency Virus.

- Subject with positive for Hepatitis B virus (HBV) antibodies and surface antigen
(including acute HBV or chronic HBV) or Hepatitis C ([HCV]; ribonucleic acid [RNA]
detected by qualitative assay). Hepatitis C RNA testing is not required in subjects
with negative Hepatitis C antibody testing.

- Subject has received a live vaccine against infectious diseases within 28 days prior
to initiation of study treatment.

- Subject has a history of drug-induced pneumonitis (interstitial lung disease) or
currently has pneumonitis.

- Subject has an infection requiring systemic therapy within 14 days prior to study drug
treatment.

- Subject has received a prior allogeneic bone marrow or solid organ transplant.

- Subject is expected to require another form of antineoplastic therapy while on study
treatment.

- Subject has had a myocardial infarction or unstable angina within 6 months prior to
the start of study treatment or currently has an uncontrolled illness including, but
not limited to symptomatic congestive heart failure, clinically significant cardiac
disease, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social
situations that would limit compliance with study requirements.

- Any condition that makes the subject unsuitable for study participation.

- Subject has had a major surgical procedure and has not completely recovered within 28
days prior to the start of study treatment.