Determined that enrollment would be limited due to the change in care setting.

This clinical trial will enroll subjects with HER2+ solid tumors and is conducted in two phases, which are phase 1a and phase 1b. The primary objective of phase 1 is to determine the safety and tolerability of AB-201 in subjects with advanced HER2+ solid tumors.

This is a Phase 1, multicenter study designed to evaluate the safety, tolerability, and efficacy of AB-201 in subjects with advanced HER2+ solid tumors (specifically, breast and gastric/GEJ cancers).

The Phase 1 will be conducted in two parts, an initial dose escalation stage followed by a dose expansion stage.

Study participation for each subject begins with up to 28 days (1 month) of screening following written informed consent, then lymphodepletion treatment, followed by AB-201. Up to 3 doses of AB-201 may be administered. All subjects will be monitored for a total duration of 18 months of follow up from the first dose administration of AB-201.

Inclusion Criteria:

• ECOG performance status 0 to 1.
• Histologically confirmed HER2 expressed breast or gastric/GEJ cancer IHC ≥ 2+ within 6 months prior to study entry.
• Confirmed diagnosis of an advanced/unresectable or metastatic HER2+ breast or gastric/GEJ cancer that is refractory to, or intolerable of standard treatment, or for which no standard treatment is available.
• Must have received prior cancer therapy: Subjects with breast cancer must have received ≥ 3 prior systemic therapies; subjects with gastric/GEJ cancer must have received ≥ 2 prior systemic therapy(ies); subjects with IHC 3+ or IHC 2+/ISH+ cancers must have received previous treatment with a HER2-targeting therapy.

Exclusion Criteria:

• Known past or current malignancy other than inclusion diagnosis.
• Known clinically significant cardiac disease.
• Active central nervous system (CNS) metastases, or involvement of the CNS, unless there is a history of at least 3 months of sustained remission.
• Unresolved toxicities from prior anticancer therapy.
• Ongoing uncontrolled systemic infections requiring antibiotic, anti-fungal, or anti-viral therapy.
• History of sensitivity or intolerance to cyclophosphamide or fludarabine.
• Currently Pregnant or lactating
• Severe disease progression or health deterioration within 2 weeks prior to lymphodepletion regimen.