A Study to Test How Well Different Doses of BI 3706674 Are Tolerated by People With Advanced Cancer in the Stomach and Oesophagus

Inclusion Criteria:

1. Patients with pathologically confirmed diagnosis of locally advanced or metastaticgastric adenocarcinoma (GAC), oesophageal adenocarcinomas (EAC), andgastroesophageal junction adenocarcinoma (GEJAC) with Kirsten rat sarcoma viraloncogene homolog (KRAS) wild type (wt) amplification and documented diseaseprogression despite at least 1 line of prior therapy. KRAS status will be confirmedretrospectively for those with a known KRAS status or determined prospectively (doseconfirmation and expansion) if KRAS status is unknown, using archival tissue (ifavailable) or a fresh biopsy. Dose escalation (Part A) only: Patients with advanced or metastatic relapsed orrefractory solid tumours of any histology with KRAS wt amplification or harbouring aKRAS G12V mutation who have exhausted treatment options known to prolong survivalfor their disease. Detection of KRAS status by a local test is allowed for enrolmentbut will be retrospectively confirmed.

2. Patients who have failed conventional treatment or for whom no therapy of provenefficacy exists or who are not eligible for established treatment options.

3. Dose confirmation (Part B) only: Patient is willing and able to undergo mandatorypre- and on-treatment low risk tumour biopsies. Patients with a high risk for biopsycomplications can be included without undergoing pre- and on-treatment tumour biopsyas long as archival tumour tissue is available for confirmation of KRAS status.

4. At least one target lesion that can be measured per RECIST version 1.1 (radiatedlesions do not qualify as target lesions unless there has been demonstratedprogression of the lesion after completion of radiotherapy) Dose escalation (Part A)only: Patients with no lesions measurable per RECIST version 1.1 may be included ifagreed between Sponsor and investigator.

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

6. All toxicities related to previous anti-cancer therapies have resolved ≤ CTCAE Grade 1 prior to trial treatment administration (except for alopecia and peripheralneuropathy which must be ≤ CTCAE Grade 2 and amenorrhea/menstrual disorders whichcan be any grade).

7. Life expectancy ≥3 months at the start of treatment in the opinion of theinvestigator.

8. Age ≥18 years of age, or over the legal age of consent as required by locallegislation.

Further inclusion criteria apply.

Exclusion Criteria:

1. Previous anti-cancer chemotherapy within 3 weeks of the first administration oftrial drug.

2. Previous anti-cancer hormonal treatment or anti-cancer immunotherapy within 2 weeksof the first administration of trial drug.

3. Previous treatment with rat sarcoma (RAS), mitogen-activated protein kinases (MAPKs)or son of sevenless homolog 1 (SOS1) targeting agents.

4. Presence of cardiovascular abnormalities such as uncontrolled hypertension (definedas systolic blood pressure ≥140 and/or diastolic blood pressure ≥90 millimetre ofmercury (mmHg)), congestive heart failure New York Heart Association (NYHA)classification of ≥ III or IV, unstable angina or poorly controlled arrhythmia.History of myocardial infarction, stroke, or pulmonary embolism within 6 monthsprior to randomisation.

5. Left ventricular ejection fraction (LVEF) <50%.

6. Congenital or family history of long QT prolongation syndrome.

7. Mean resting corrected QT interval (QTcF) >470 msec.

8. Radiotherapy within 2 weeks prior to start of treatment, except as follows:

• Palliative radiotherapy to regions other than the chest is allowed if completedat least 2 weeks prior to randomisation.

• Single dose palliative radiotherapy for symptomatic metastasis within 2 weeksprior to randomisation may be allowed but must be discussed with the Sponsor.

Further exclusion criteria apply.