Pharmaceutical Science: Research Trends and Challenges Vol. 2

Breast cancer pharmacotherapy has shifted from histology-led cytotoxic treatment towards molecularly stratified, temporally adaptive therapy. This critical narrative review evaluates the pharmacological architecture of breast cancer treatment, integrating endocrine therapy, cytotoxic chemotherapy, human epidermal growth factor receptor 2 (HER2)-directed agents, cyclin-dependent kinase 4/6 inhibitors, phosphoinositide 3-kinase–AKT–mammalian target of rapamycin pathway inhibitors, poly(ADP-ribose) polymerase inhibitors, immune checkpoint inhibitors and antibody–drug conjugates. Particular attention is given to how biomarker context, disease setting and prior selective pressure determine treatment benefit, resistance and toxicity. Literature published from January 2000 through 5 July 2026 was identified from open scholarly sources and critically appraised for methodological quality, clinical relevance, consistency and claim–source alignment. The evidence supports several robust conclusions. Endocrine therapy remains the backbone of hormone receptor-positive disease, but its effectiveness increasingly depends on rational combinations and resistance-genotype selection. HER2-targeted treatment has progressed from receptor blockade to highly active antibody–drug conjugates, with therapeutic activity extending into HER2-low and HER2-ultralow disease while creating new diagnostic and pulmonary-toxicity challenges. In triple-negative breast cancer, immunotherapy benefits biomarker- and stage-defined populations, whereas Trop-2-directed conjugates are moving earlier in the treatment course. Germline BRCA1/2 status identifies a clinically validated vulnerability to poly(ADP-ribose) polymerase inhibition. Across subtypes, resistance is not a single endpoint but an evolutionary process involving target alteration, pathway bypass, lineage plasticity, drug trafficking and payload resistance. The principal unresolved problem is therefore no longer whether targeted drugs work, but how to sequence, combine and adapt them without exhausting therapeutic index. Future progress will depend on prospective molecular monitoring, better cross-resistance models, standardised low-range HER2 assessment, central nervous system-inclusive trials and comparative strategies that measure survival, quality of life and cumulative toxicity rather than short-term tumour control alone.