7.2.3.3　Polymer microneedles

Microneedle technology is a method of enhancing transdermal drug delivery and has received a lot of attentions for nearly 25 years.Microneedles are micron-sized needles with a general length of 100-1000μm，which can penetrate vertically into the epidermal layer of the skin.When the microneedles pass through the cuticle layer，aqueous holes can be created，which can significantly increase drug permeability.The types of microneedles are mainly divided into solid microneedles，coated microneedles，hollow microneedles and soluble microneedles.

Solid microneedles can be used to pretreat the skin to form a catheter，thus improving drug permeability，while coated microneedles deliver the drug on the surface of the microneedles to the skin.Hollow microneedles can be used to infuse drugs into the skin through a microneedle hollow.Soluble microneedles are generally soluble polymer microneedles that encapsulate the drug and completely dissolve in the skin.In recent years，microneedles have demonstrated significant advantages in transdermal immune regulations：①small size；②painless and minimally invasive；③lower risk of infection from microbial invasion；④versatile and precise delivery to epidermis；⑤convenient，do not need professional training for management，improve patient compliance and affordability.Solid microneedles，coated microneedles and hollow microneedles have been studied in the field of immunotherapy for malaria，measles，influenza，AIDS and other diseases.Soluble microneedles，especially soluble polymer microneedles are currently widely investigated in melanoma immunotherapy.

In 2013，Kissenpfennig et al.used soluble methyl vinyl ether-maleic anhydride copolymer as microneedle matrixes to deliver antigen-loaded PLGA NPs to the continuous DCs network of the skin.After in-situ antigen uptake，cutaneous DCs delivers antigenloaded PLGA NPs to cutaneous drainage lymph nodes，which significantly improves T cells activation and proliferation.Moreover，effective antigen-specific cellular immune response can be produced in mice producing antigen-specific toxic CD8+T lymphocytes，which can effectively inhibit the growth of melanoma.Furthermore，the antigens loaded in PLGA NPs can be retained in the skin layer to improve the stability of antigens.(https://www.daowen.com)

Since 2016，Gu's research group has constructed different microneedles based on hyaluronic acid（HA）for melanoma immunotherapy.They found that the controllable release mechanism plays an important role in improving the efficacy of immunotherapy for melanoma with significant reduction of toxic and side effects.Firstly，Gu et al.developed a kind of HA microneedles loaded with pH-sensitive dextran NPs encapsulating with glucose oxidase（GOx），catalase（CAT）and aPD-1（Figure 7-7）.GOx converted blood glucose and oxygen into gluconic acid and hydrogen peroxide.The hydrogen peroxide produced by the above reaction was decomposed under the action of CAT，which further promoted the degradation of glucose and realized the controlled and continuous release of aPD-1 from dextran NPs.Compared with intratumoral injection of the same dose of aPD-1 and nondegradation microneedles，these HA microneedles can effectively activate the anti-melanoma immunity.In the same year，Gu et al.constructed a synergistic immunotherapy strategy based on HA microneedles.Inhibitor 1-MT of immunosuppressant enzyme IDO was linked to HA through covalent bond to form an amphiphilic structure，which formed NPs through self-assembling and loaded aPD-1.Subsequently，HA microneedles loaded with NPs were prepared，and accumulated into the cutaneous DCs network around local melanoma through the corneum，thus improving the retention time of the drug in the tumor site and alleviating the toxic side effects caused by systemic administration.Due to the function of hyaluronidase in TME，HA was degraded to release aPD-1 and 1-MT.The synergistic effect of aPD-1 and 1-MT can induce a strong and persistent anti-melanoma immune response.In 2017，Gu et al.added melanin and melanoma total antigen vaccines to HA microneedles.Under the irradiation of near-infrared light，the heat was generated locally，which could further improve the phagocytosis of antigens by DCs，increase the infiltration of polarized T cells and local cytokines secretion，therefore activating stronger anti-tumor vaccine effect.In 2019，Hahn et al.used HA microneedles to load HA grafted with cytotoxic T cell epitope peptides，which could effectively improve the responses of specific CTLs and inhibit the growth of melanoma.

In 2017，Jewell et al.demonstrated PLA microneedle array coated with alternate adsorption with positively charged melanoma antigen and negatively charged immunoadjuvant CpG through layer-by-layer self-assembling.The polyelectrolyte multilayers could be up to 128 layers with a thickness of about 200 nm and effectively promote the proliferation of tumor-specific CD8+T lymphocytes in vivo.In 2018，Lee et al.demonstrated DNA nanovaccine based on polycarbonate microneedle array，which was obtained by the layer-bylayer self-assembly of ultra-pH-responsive OSM-（PEG-PAEU）and poly（I：C）.The codelivery of immune adjuvant poly（I：C）and plasmid expressing ovalbumin（pOVA）can effectively inhibit the growth of melanoma cells and lung metastasis in vivo.In 2020，Lee et al.constructed a cocktail of polypeptides nanocomplex microneedle，the microneedle used cationic polypeptides and PEG as the matrixes and highly transfected cationic amphiphilic binder to co-deliver immunoadjuvant poly（I：C）and pOVA，which realized the immunotherapy of melanoma.Wu et al.constructed a core-shell microneedle to deliver aPDL1 and 1-MT for immunotherapy of melanoma.Through strong electrostatic interaction，the aPD-L1 was adsorbed on the shell and concentrated in the tip of the microneedle.The crystallization of 1-MT was inhibited by the hydrogen bond interaction between PVA and 1-MT，so that 1-MT was supersaturated to form homogeneous drug-polymer solution and loaded in homogeneous microneedles with high drug load.PVA，as the core of the microneedles with abundant hydrogen bonds，inhibits its crystallization through hydrogen bond interaction with 1-MT，so that 1-MT is supersaturated to form homogeneous drugpolymer solution and loaded in homogeneous microneedles with high drug loading efficiency.Compared with intratumoral injection with the same dose，microneedles with core-shell structure show better melanoma inhibition efficiency.