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Short pharmaceutical active oligonucleotides such as small interfering RNA (siRNA) or cytidine‐phosphate‐guanosine (CpG) are considered as powerful therapeutic alternatives, especially to medicate hard‐to‐treat diseases (e.g., liver fibrosis or cancer). Unfortunately, these molecules are equipped with poor pharmacokinetic properties that prevent them from translation. Well‐defined nanosized carriers can provide opportunities to optimize their delivery and guide them to their site of action. Among several concepts, this Feature Article focuses on cationic nanohydrogel particles as a universal delivery system for small anionic molecules including siRNA and CpG. Cationic nanohydrogels are derived from preaggregated precursor block copolymers, which are further cross‐linked to obtain well‐defined nanoparticles of tunable sizes and with (degradable) cationic cores. Novel opportunities for oligonucleotide delivery in vitro and in vivo with respect to liver fibrosis therapies will be highlighted as well as perspectives toward modulating the immune system. In general, the approach of covalently stabilized cationic carrier systems can contribute to find advanced oligonucleotide therapeutics.

Immunosuppressive M2 macrophages govern the immunophathogenic micromilieu in many severe diseases including cancer or fibrosis, thus, their re‐polarization through RNA interference is a promising concept to support combinatorial therapies. For targeted siRNA delivery, however, safe and stable carriers are required that manage cell specific transport to M2 macrophages. Here, siRNA‐loaded cationic nanogels are reported with α‐mannosyl decorated surfaces that target and modify M2 macrophages selectively. Via amphiphilic precursor block copolymers bearing one single α‐mannosyl moiety at their chain end mannosylated cationic nanohydrogel particles (ManNP) were obtained of 20 nm diameter determined by dynamic light scattering and cryogenic electron transmission microscopy. α‐Mannosyl surface modification is confirmed by agglutination with concanavalin A. SiRNA‐loaded ManNP preferentially targets the overexpressed mannose receptor CD206 on M2 macrophages, as shown by in vitro cell uptake studies in M2 polarized primary macrophages. This specificity is confirmed, since ManNP uptake could be reduced by blocking of CD206 with mannan. Effective ManNP‐guided siRNA delivery is confirmed by sequence‐specific gene knockdown of CSF‐1R in M2‐type macrophages exclusively, while the expression levels in M1‐polarized macrophages is not affected. In conclusion, α‐mannosyl‐functionalized ManNPs are promising universal siRNA carriers for targeted immunomodulatory treatment of immunosuppressive macrophages.

Macrophages are the front soldiers of the innate immune system and are vital for immune defense, tumor surveillance, and tissue homeostasis. In chronic diseases, including cancer and liver fibrosis, macrophages can be forced into an immunosuppressive and profibrotic M2 phenotype. M2-type macrophages overexpress the mannose receptor CD206. Targeting these cells via CD206 and macrophage repolarization towards an immune stimulating and antifibrotic M1 phenotype through RNA interference represents an appealing therapeutic approach. We designed nanohydrogel particles equipped with mannose residues on the surface (ManNP) that delivered siRNA more efficiently to M2 polarized macrophages compared to their untargeted counterparts (NonNP) in vitro. The ManNP were then assessed for their in vivo targeting potential in mice with experimental liver fibrosis that is characterized by increased profibrotic (and immunosuppressive) M2-type macrophages. Double-labelled siRNA-loaded ManNP carrying two different near infrared labels for siRNA and ManNP showed good biocompatibility and robust uptake in fibrotic livers as assessed by in vivo near infrared imaging. siRNA–ManNP were highly colocalized with CD206+ M2-type macrophages on a cellular level, while untargeted NP (NonNP) showed little colocalization and were non-specifically taken up by other liver cells. ManNP did not induce hepatic inflammation or kidney dysfunction, as demonstrated by serological analysis. In conclusion, α-mannosyl-functionalized ManNP direct NP towards M2-type macrophages in diseased livers and prevent unspecific uptake in non-target cells. ManNP are promising vehicles for siRNA and other drugs for immunomodulatory treatment of liver fibrosis and liver cancer.

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Front Cover: Targeting immunosuppressive M2 macrophages for siRNA‐mediated repolarization towards immunoactive M1 macrophages is achieved by α‐mannosylfunctionalized cationic nanohydrogel particles. These particles are prepared from amphiphilic reactive ester block copolymers obtained by RAFT polymerization. A novel α‐mannosyl‐chain‐transfer‐agent guarantees selective α‐chain end group polymer functionalization. Subsequent self‐assembly and cross‐linking affords α‐mannosyl‐functionalized cationic nanohydrogels that complex and deliver siRNA to M2 macrophages. This is reported by Nadine Leber, Leonard Kaps, Aiting Yang, Misbah Aslam, Mariacristina Giardino, Adrian Klefenz, Niklas Choteschovsky, Sebastian Rosigkeit, Asmaa Mostafa, Lutz Nuhn, Detlef Schuppan, and Rudolf Zentel in article 1900162.

This paper deals with a concept to improve the loading capability of cationic nanohydrogel particles with siRNA. For this purpose, a new tetrafunctional crosslinker is synthesized via a peptide coupling approach using lysine and spermine derivatives. Applying this four‐arm crosslinker to the particle synthesis makes it possible to perform the crosslinking with an excess or a deficit of the crosslinker. This allows for varying numbers of cationic groups per cationic core and its crosslinking density while the stability of the carrier system remains. The obtained cationic nanohydrogel particles are narrowly distributed in size as determined by dynamic light scattering measurements. Zeta potential measurements confirm the cationic nature of these carriers (ζ > +30 mV) and the ability to form complex anionic siRNA (agarose gel electrophoresis). As a result, it becomes possible to increase the siRNA loading by a factor of four by varying the composition of the crosslinked core. Such an increased siRNA loading should lead to an improved therapeutic gene knockdown effect.

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