Targeting immunosuppressive metastatic cancer cells is a key challenge in therapy. We recently have shown that a rigid‐rod aromatic, pBP−NBD, that responds to enzymes and kill immunosuppressive metastatic osteosarcoma (mOS) and castration resistant prostate cancer (CRPC) cells in mimetic bone microenvironment. However, pBP−NBD demonstrated moderate efficacy against CRPC cells. To enhance activity, we incorporated the unnatural amino acid L‐ or D‐4,4′‐biphenylalanine (L‐ or D−BiP) into pBP−NBD, drastically increasing cellular uptake and CRPC inhibition. Specifically, we inserted BiP into pBP−NBD to target mOS (Saos2 and SJSA1) and CRPC (VCaP and PC3) cells with overexpressed phosphatases. Our results show that the D‐peptide backbone with an aspartate methyl diester at the C‐terminal offers the highest activity against these immunosuppressive mOS and CRPC cells. Importantly, imaging shows that the peptide assemblies almost instantly enter the cells and accumulate primarily within the endoplasmic reticulum of Saos2, SJSA1, and PC3 cells and at the lysosomes of VCaP cells. By using BiP to boost cellular uptake and self‐assembly within cancer cells, this work illustrates an unnatural hydrophobic amino acid as a versatile and effective residue to boost endocytosis of synthetic peptides for intracellular self‐assembly. This research demonstrates that the integration of unnatural amino acids (L‐ or D‐4,4′‐biphenylalanine) into peptide assemblies significantly accelerates their accumulation within prostate cancer cells, boosting their capacity to hinder cancer proliferation. This work paves a way for engineering supramolecular medicine, potentially heightening the efficacy of peptide‐based treatments for prostate cancer. It may lead to more effective therapeutic strategies for treating other cancers.