## 5 Therapeutic Properties of Nanoparticles in Breast Cancer Treatment
### Enzyme-Responsive Nanoparticles  
Engineered nanoparticles responsive to enzymes are designed to selectively interact with particular enzymes present in tumor tissues [81]. This leads to the precise delivery of anticancer drugs, minimizing systemic side effects and improving treatment outcomes. Additionally, these nanoparticles effectively engage with internal enzymes under mild conditions, such as near-neutral pH levels, water-based environments, and low temperatures [82]. Typically, tumor cells exhibit higher concentrations of gelatinase, a proteolytic enzyme, compared to normal tissues [83]. In this regard, a chitosan/gelatin hybrid nanogel incorporating gold nanoparticles (CS/AuNPs@Gel-Dox nanogel) was designed as an enzyme-responsive nanoparticle platform for cancer therapy [84]. The zeta potential and the size of the CS/AuNPs were influenced by cross-linking. Upon exposure to gelatinase, the nanogel's structure undergoes enzyme-specific degradation, enabling targeted drug delivery from the CS/AuNPs@Gel-Dox nanogel [84]. The nanogel's compatibility with cells and its efficacy as a cancer drug carrier were validated through cytotoxicity tests. Flow cytometry results indicated that the CS/AuNPs@Gel-Dox nanogel was successfully absorbed by cells. Therefore, this tailor-made, enzyme-responsive nanogel holds promise for targeted treatment of various solid tumors [84]. Similar research was carried out by [PERSON] et al. with disulfiram and doxorubicin conjugated with polymeric nanoparticles [85]. Another interesting study was recently conducted with peptide conjugate crafted to combine mild photothermal therapy with immunotherapy in a unified nanosystem for treating breast cancer. The peptide-photosensitizer conjugate nanospheres have a distinctive property: they release the PD-L1 (programmed cell death-ligand 1) antagonist peptide in response to MMP (matrix metalloproteinase) and undergo a coassembled morphology change. Both of these characteristics were studied both in vitro and in vivo [86]. With laser irradiation, these PPC nanospheres demonstrate impressive antitumor effects, efficiently curbing the growth of localized tumors, distant tumors, and in vivo lung metastases [86]. The PPC (peptide-photosensitizer conjugate) is modular in design, allowing for adjustments in its responsiveness, the delivery speed of immune checkpoint antagonists, and the photosensitizer's aggregated state for different uses. [PERSON] et al., in their study, noted the localized enzyme response and carrier transformation through changes in fluorescence intensity [86]. A drug delivery system (POL-MSN) was developed that responds to both pH and esterase. This system was created by enclosing Dox-loaded MSNs with poly (\(\beta\)-amino ester) and boronate esters. Given the acidic pH and elevated esterase levels in cancer cells, POL-MSN is designed to smartly discharge its drug payload at the tumor location, targeting and eliminating the cancer cells. This system has demonstrated potent cytotoxic effects against MDA-MB-231 human breast cancer cells [87]. In addition, microbots that are fabricated using nanomaterials are gaining attention among researchers for the treatment of specific cancer types via targeted and controlled drug delivery [88; 89; 90]. Table 1 provides a concise overview of nanotherapeutics for breast cancer investigated in prior studies, detailing the delivery techniques employed and their respective findings.