Introduction: Small cell lung cancer (SCLC) represents an aggressive malignancy that initially responds well to frontline platinum-based chemotherapy. However, most patients relapse quickly due to rapid development of chemoresistance, emphasizing the urgent need for novel therapeutic strategies to achieve durable clinical benefit. Methods: To elucidate the mechanisms driving treatment failure, we established in vitro SCLC models that acquired cisplatin resistance across distinct molecular subtypes. These models were characterized by proliferation, cell cycle, and proteomic assays. Drug screens were conducted to identify therapeutic vulnerabilities, including responses to unconjugated antibody-drug conjugate (ADC) payloads and targeted pathway inhibitors. Results: Cisplatin-resistant (CR) SCLC models exhibited broad cross-resistance to clinically relevant cytotoxic drugs such as carboplatin, lurbinectedin, and topoisomerase I and II (TOP1 and TOP2) inhibitors. Resistance was associated with diminished intracellular platinum accumulation, reduced proliferation, and altered cell cycle distribution with G2/M arrest or senescent-like features compared to the corresponding sensitive parental lines. Proteomic analysis of CR models, together with patient-derived samples at treatment relapse, revealed extensive metabolic reprogramming and downregulation of DNA repair and checkpoint control pathways, highlighting shared adaptive signatures. Screening of unconjugated ADC payloads uncovered mechanism-specific vulnerabilities. While CR models were resistant to DNA-damaging payloads such as calicheamicin, microtubule-disrupting ADC payloads (MMAE and DM1) retained sensitivity across all CR models. Responses to TOP1-directed ADC payloads varied and were associated with SLFN11 expression levels. Importantly, inhibition of ATR with ceralasertib synergized with DNA-damaging ADC payloads, re-sensitizing SLFN11-low resistant models by increasing DNA damage and inducing apoptosis. Conclusion: Our findings demonstrate shared adaptive mechanisms of acquired cisplatin resistance in SCLC, including checkpoint rewiring and metabolic flexibility. SLFN11 was identified as a key determinant of TOP1-directed ADC efficacy. Combining ATR inhibition with DNA-damaging ADC payloads represents a promising strategy to restore DNA damage-induced cell death in resistant SCLC. These insights provide a translational framework for developing rational ADC-based combination therapies to overcome chemoresistance in relapsed SCLC.