Arm 1: AZD0901 arm 1 IV, Q3W Arm 2: AZD0901 arm 2 IV, Q3W Arm 3: Participants in the Investigator's choice arm, including regionally accepted chemotherapies or targeted therapies 2L: Ramucirumab 8 mg/kg IV on Days 1 and 15 and paclitaxel 80 mg/m2 IV on Days 1, 8, and 15, Q4W 2L: Paclitaxel 80 mg/m2 IV on Days 1, 8, and 15, Q4W (for participants with contraindication to ramucirumab only) 2L: Docetaxel 75-100 mg/m2 IV on Day 1, Q3W (for participants with contraindication to ramucirumab only) 3L+: Irinotecan 150-180 mg/m2 IV on Days 1 and 15, Q4W 3L+: TAS-102 35 mg/m2 up to a maximum of 80 mg orally twice a day on Days 1 to 5 and Days 8 to 12, Q4W (except China) 3L+: Apatinib 500-850 mg, orally once daily, Q4W (China only) primaryPurpose: TREATMENT masking: SINGLE maskingDescription: This is an open-label study; however, it will be conducted 'sponsor-blind' and the specific treatment to be taken by a participant will be assigned using an IRT/RTSM. To maintain the integrity of the study, sponsor access to treatment records will be restricted, and, in particular, under no circumstances will the sponsor undertake any efficacy analysis by treatment arm during the study. A Study Integrity Plan will be generated in which nominated individuals who will be granted access to any treatment-revealing data will be pre-specified, with their reason for requiring access detailed. whoMasked: OUTCOMES_ASSESSOR count: 589 type: ESTIMATED name: AZD0901 name: AZD0901 name: Ramucirumab+ paclitaxel name: Paclitaxel name: Docetaxel name: Irinotecan name: TAS-102 name: Apatinib measure: Progression Free Survival (PFS) in all randomized participants measure: Overall Survival (OS) for 3L+ participants measure: OS in all randomized participants measure: PFS for 3L+ participants measure: Objective Response Rate (ORR) in all randomized participants measure: ORR for 3L+ participants measure: Duration of Response (DoR) in all randomized participants measure: Serum concentrations of AZD0901, total antibody and MMAE measure: Status of ADA to AZD0901 measure: Incidence of Treatment-Emergent Adverse Events [Safety and Tolerability]. Changes from baseline in vital signs, clinical laboratory results, and ECGs measure: PK parameters (such as peak concentration, as data allow) of AZD0901, total antibody and MMAE measure: PK parameters (such as trough concentration, as data allow) of AZD0901, total antibody and MMAE measure: Prevalence and incidence of ADA to AZD0901 measure: Titer of ADA to AZD0901 sex: ALL minimumAge: 18 Years maximumAge: 130 Years stdAges: ADULT stdAges: OLDER_ADULT facility: Research Site status: NOT_YET_RECRUITING city: Duarte state: California zip: 91010 country: United States lat: 34.13945 lon: -117.97729 facility: Research Site status: WITHDRAWN city: Santa Rosa state: California zip: 95403 country: United States lat: 38.44047 lon: -122.71443 facility: Research Site status: NOT_YET_RECRUITING city: Atlanta state: Georgia zip: 30322 country: United States lat: 33.749 lon: -84.38798 facility: Research Site status: NOT_YET_RECRUITING city: Marietta state: Georgia zip: 30060 country: United States lat: 33.9526 lon: -84.54993 facility: Research Site status: WITHDRAWN city: Baltimore state: Maryland zip: 21287 country: United States lat: 39.29038 lon: -76.61219 facility: Research Site status: NOT_YET_RECRUITING city: Boston state: Massachusetts zip: 02114 country: United States lat: 42.35843 lon: -71.05977 facility: Research Site status: NOT_YET_RECRUITING city: Kansas City state: Missouri zip: 64111 country: United States lat: 39.09973 lon: -94.57857 facility: Research Site status: WITHDRAWN city: Saint Louis state: Missouri zip: 63110 country: United States lat: 38.62727 lon: -90.19789 facility: Research Site status: NOT_YET_RECRUITING city: Saint Louis state: Missouri zip: 63128 country: United States lat: 38.62727 lon: -90.19789 facility: Research Site status: NOT_YET_RECRUITING city: Mineola state: New York zip: 11501 country: United States lat: 40.74927 lon: -73.64068 facility: Research Site status: NOT_YET_RECRUITING city: New York state: New York zip: 10016 country: United 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country: Canada lat: 42.98339 lon: -81.23304 facility: Research Site status: NOT_YET_RECRUITING city: Ottawa state: Ontario zip: K1H 8L6 country: Canada lat: 45.41117 lon: -75.69812 facility: Research Site status: NOT_YET_RECRUITING city: Toronto state: Ontario zip: M5G 2M9 country: Canada lat: 43.70011 lon: -79.4163 facility: Research Site status: NOT_YET_RECRUITING city: Beijing zip: 100210 country: China lat: 39.9075 lon: 116.39723 facility: Research Site status: NOT_YET_RECRUITING city: Changsha zip: 410013 country: China lat: 28.19874 lon: 112.97087 facility: Research Site status: RECRUITING city: Chengdu zip: 610000 country: China lat: 30.66667 lon: 104.06667 facility: Research Site status: NOT_YET_RECRUITING city: Chengdu zip: 610042 country: China lat: 30.66667 lon: 104.06667 facility: Research Site status: NOT_YET_RECRUITING city: Fuzhou City zip: 350001 country: China facility: Research Site status: NOT_YET_RECRUITING city: Fuzhou zip: 350014 country: China lat: 26.06139 lon: 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