Cancer chemotherapy has experienced fast developments during the last decades, but the current clinical data revealed only modest therapeutic efficacy, partially due to inadequate drug accumulation and penetration in tumors [1], [2]. In this regard, liposomes, gold nanoparticles and polymeric micelles have emerged as an attractive approach for targeted delivery of therapeutic agents to tumors, accompanied by prolonged circulation time and reduced toxicity [3], [4]. The tumor-targeting effect of nanoparticles (NPs) can be derived from the size-determined enhanced permeability and retention (EPR) effect, or the biorecognition-based active-targeting bioactive antibodies [5], [6], peptides [7], [8], oligonucleotide aptamers [9], [10] and small molecules [11]. Nevertheless, reports have indicated that on average, only 0.7% of the injected NPs can reach tumors [12]. Even for the drug-loaded NPs that have already aggregated around solid tumors, further penetration into the deep tumor tissues can still be challenging, resulting in suboptimal therapeutic efficacy.
In addition to strategies that enhance the tumor penetration of NPs, such as enzyme-triggered activation and remodeling of the tumor microenvironment [13], [14], [15], the intrinsic physical properties of NPs, specifically their size and shape, are recognized as critical determinants of successful targeted drug delivery and intratumoral distribution [16], [17], [18]. Generally, smaller nanoparticles penetrate tumors more deeply than large ones, but suffer from rapid clearance and poor retention [19], [20]. Meanwhile, NPs in the 100–200 nm range are typically known for their good accumulation at tumor sites [21]. Moreover, owing to the distinct hemodynamics and larger interacting areas on cell surfaces, nanorods often exhibit better cell internalization and deeper penetration in tumor tissues compared to nanospheres [22], [23], [24]. However, excessively long nanorods with higher aspect ratios (ARs) tend to exhibit reduced permeability [25]. Therefore, size-switching strategy has been developed for smart nanomedicine, where larger nanoparticles transform into smaller or rod-like shapes upon cellular uptake, leading to increased tumor penetration depth [26], [27]. However, these findings are highly dependent on both NP composition and tumor type. Apart from the most frequently studied NPs, such as gold nanoparticles [28], [29], PEG-PLA copolymers [30], and nano silica [31], the impacts of supramolecular NPs self-assembled from simple organic components have not been thoroughly investigated.
N-fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF), as the simplistic peptide capable for supramolecular self-assembly, has been extensively used as the core structure for constructing various hydrogel materials [32], [33]. Upon the combined hydrophobic interaction and β-sheet stacking, Fmoc-FF-based biomaterials have found broad applications in wound dressing, drug delivery, cell culture, biosensing, etc., but prevalently in the format of hydrogels [34], [35], [36]. We have previously reported a novel type of co-assembled NPs based on the Fmoc-FF scaffold with tunable shapes and sizes [37], [38]. Among spherical, rod-like, and fibrous NPs co-assembled from Fmoc-FF and Fmoc-monosaccharides, long nanofibers showed better functional mimicking ability of natural heparin. In addition, the supramolecular polymerization process provides enhanced multivalent effect [39], [40], [41], [42], thereby improving targeting efficiency upon incorporation of anchoring saccharide moieties. These results prompted us to further explore these nanostructures as drug carriers for targeted delivery of anti-tumor drug doxorubicin (DOX).
In the current study, Fmoc-functionalized carbohydrate targeting components and the hydrophobic anti-tumor drug DOX were co-assembled into the Fmoc-FF template (Fig. 1), which is known to form functional nanomedicines with shapes ranging from nanospheres to nanorods of different aspect ratios. Glucose, galactose and fucose were used as the carbohydrate targeting moiety [43], [44], while galactose possess known additional affinity towards hepatocytes [45], [46]. Moreover, anti-tumor drug DOX was incorporated into the nano-assembly structures through H-bonding, π-π stacking and hydrophobic interactions. One of the advantages of the unprecedented supramolecular design described here using simple molecules mixtures of variable composition is the precise synergetic control of the nanoparticle shape and the multivalent presentation of surface glyco-ligands, thereby enabling strong in vivo targeting affinity. Thus, the therapeutic effectiveness of the DOX-loaded nanospheres/nanorods can be evaluated using an in vitro 3D tumor spheroid model and an in vivo tumor-bearing mice model, exploring the influences of nanocarrier shapes from self-assembled Fmoc-FF scaffold on tumor penetration, biodistribution and anti-tumor performances.