gemcitabine + nab-paclitaxel + paricalcitol + PD-1 antibody

This is a four-drug combination regimen consisting of gemcitabine, nab-paclitaxel, paricalcitol, and a PD-1 antibody. - **Gemcitabine** is a nucleoside analog used as chemotherapy that inhibits DNA synthesis by incorporation into DNA and inhibition of ribonucleotide reductase. - **Nab-paclitaxel** (nanoparticle albumin-bound paclitaxel) is a microtubule inhibitor formulated with albumin to improve solubility and delivery; it disrupts mitosis in rapidly dividing cells. - **Paricalcitol** is a synthetic vitamin D analog that acts as an agonist at the vitamin D receptor, primarily used to manage secondary hyperparathyroidism but also investigated for anti-tumor effects via modulation of cell proliferation and immune response. - **PD-1 antibody** refers to monoclonal antibodies targeting programmed cell death protein 1 (PDCD1), blocking its interaction with ligands PD-L1/PD-L2 to enhance T-cell mediated anti-tumor immunity (checkpoint inhibition). This combination aims to leverage cytotoxic chemotherapy (gemcitabine/nab-paclitaxel), immunomodulation (paricalcitol), and immune checkpoint blockade (PD-1 antibody) for synergistic anti-cancer activity, particularly in pancreatic cancer.