Trilaciclib hydrochloride is primarily used in the treatment of small cell lung cancer (SCLC). It is administered intravenously to patients undergoing chemotherapy to protect bone marrow cells from damage. By inhibiting cyclin-dependent kinase 4/6 (CDK4/6), it helps reduce chemotherapy-induced myelosuppression, a condition where bone marrow activity is decreased, leading to lower production of blood cells. This application helps in maintaining healthier blood cell counts, reducing the need for interventions like blood transfusions or growth factor support, and improving overall patient tolerance to chemotherapy.