**6. Therapeutic strategies and future challenges to regulate mCRP in breast cancer**

Here we have exposed that the function of mCRP on complement has been widely explored. But these proteins also can regulate non-complement signaling in promoting cancer proliferation, chemoresistance, and metastasis. We proposed a model of signaling pathways activated by mCRP [12] and recently Bharti et al. also proposed a series of pathways intracellularly by CD55 [43]. Due to their relevance in the potential of anticancer antibodies, they have been proposed and studied mCRP as therapeutic targets through various models and strategies: small interfering RNAs (siRNA) [69, 71, 84, 85], antibodies anti-mCRP [13], and enzyme-peptide [13, 86]. Although there are multiple studies in the preclinical stage and in development, none are exclusive for each mCRP and neither has reached clinical use.

Geis et al. designed siRNAs for post-transcriptional gene knockdown of CD46, CD55, and CD59 aiming to sensitize tumor cells lines (BT474 (breast) and K562 (erythroleukemia) and Du145 (prostate)) to better for tumor cell destruction by complement. Interestingly, the breast carcinoma cells BT474 were predominantly sensitized to CDC upon inhibiting CD46 expression. In contrast, suppression of CD55 and CD59 had no or only a minor effect, suggesting that CD46 is more critical in regulating complement activity [69]. But it is necessary to identify the activity intracellular to all mCRP in this context.

Other authors also used siRNAs anti-mCRP, but the delivery of chemically was stabilized siRNAs using cationic lipoplexes (AtuPLEXes). Their results suggest that siRNA-induced inhibition of mCRP expression enhances complement and macrophage-mediated anti-tumor activity of trastuzumab and pertuzumab on HER2 positive tumor cells [71].

To increase the selectivity of silencing, other authors used siRNAs encapsulated in transferrin-coupled lipoplexes for the specific targeted and delivery to transferrin receptor CD71high expressing BT474 tumor cells. The mCRP knockdown led to a significant increase of CDC in BT474; it was also observed that the downregulation of CD46 and CD55 significantly increased C3 opsonization in these tumor cells [84].

The inhibition of mCRP with siRNA has been used to study the relevance of this molecule in other tumors [87, 88]. In general, silencing can sensitize tumor cells to CDC and ADCC *in vitro*. Although siRNA is an attractive strategy, in vivo data will be needed to validate the therapeutic potential.

Wang et al. explored three different strategies to inhibit the activity of mCRP. One strategy consisted of inhibiting the expression of mCRP using shRNA (short hairpin RNA). Other approach blocked the function of CD55 and CD59 using targeted monoclonal antibodies; the third consisted of treatment with a phosphatidylinositolspecific phospholipase C (PI-PLC), which caused a significant decrease in the surface area of CD55 and CD59. These strategies significantly improved cell lysis of SK-BR-3 and BT474 cells with trastuzumab [13].

The use of monoclonal antibodies anti-mCRP or bispecific antibodies (bsAbs) has also been evaluated in other types of cancer, demonstrating that the efficacy of therapeutic antibodies can be increased by blocking these proteins [89–96].

Ad35K++ that binds with high affinity to CD46 and is one of the most advanced strategies to block CD46 activity. This peptide has been evaluated in lymphoma model and the preclinical studies Ad35K++ have been demonstrated safety and efficacy. Intravenous Ad35K++ injection triggers the shedding of the CD46 extracellular *Membrane-Bound Complement Regulatory Proteins in Breast Cancer: Are They Best… DOI: http://dx.doi.org/10.5772/intechopen.109945*

domain in xenograft mouse tumor models and in macaques. The authors suggest their study is the basis for an investigational new drug application for the use of Ad35K++ in combination with rituximab in the treatment of patients with B-cell malignancies [86, 97]. The first studies with this peptide were evaluated in cancer lines cells (Raji lymphoma cells and BT474-M) using alemtuzumab and trastuzumab, in both case increased CDC [86].
