Recent nanomedicine articles of outstanding interest

Nanoparticles as cancer stem cell-specific inhibitors

Evaluation of: Liu Y, Chen C, Qian P et al. Gd-metallofullerenol nanomaterial as non-toxic breast cancer stem cell-specific inhibitor. Nat. Commun. 6:5988 (2015).

To date, nanomaterials have mostly been considered as carriers for existing therapeutic agents. However, in an interesting new study, Li et al. have provided evidence that the metallofullerenol nanomaterial Gd@C₈₂(OH)₂₂ possesses intrinsic activity against breast cancer stem cells (CSCs) [1]. In other words, these nanoparticles were found to act as drugs per se. The fullerene-based nanomaterial Gd@C₈₂(OH)₂₂ is comprised of a rare earth atom gadolinium encased in a cage of 82 carbon atoms; the surface of the carbon cage is modified with 22 hydroxyl groups to form a virus-like particle, Gd@C₈₂(OH)₂₂. These nanoparticles were previously shown to exert pronounced anticancer effects [2]. However, when the authors exposed so-called triple-negative breast cancer cell lines, MDA-MB-231 and BT549, enriched for features associated with epithelial-to-mesenchymal transition (EMT) and breast CSC phenotypes, to Gd@C₈₂(OH)₂₂ they could not detect any effects on cell proliferation or apoptosis while morphological changes suggestive of a reversal of the EMT phenotype were observed. This was supported by an increased expression of epithelial markers and reduced expression of mesenchymal markers at the mRNA and protein level. The authors also investigated the effects of Gd@C₈₂(OH)₂₂ on triple-negative breast cancer cell behavior in vivo using nude mice injected subcutaneously (sc.) with MDA-MB-231 cells. The Gd@C₈₂(OH)₂₂ nanoparticles were administrated by intraperitoneal (ip.) injection once daily and were shown to significantly inhibit tumor growth in this model. Moreover, the tumors derived from the Gd@C₈₂(OH)₂₂-treated mice remained well confined and noninvasive when compared with control animals and the authors also found that lung and liver metastases were significantly reduced. In an attempt to elucidate the underlying mechanisms, Li et al. performed next-generation sequencing of MDA-MB-231 cells exposed to Gd@C₈₂(OH)₂₂ nanoparticles. Treatment with 50 μM of particles for 21 days significantly reduced the expression of genes associated with the mesenchymal phenotype as well as those associated with CSCs. Furthermore, the authors provided evidence that the metallofullerenol nanoparticles blocked EMT through inhibition of HIF-1α and TGF-β signaling and it was speculated that this occurred due to the efficiency of the nanoparticles to scavenge reactive oxygen species, which are known to be potent stimulators of HIF-1α and TGF-β expression. Notably, under hypoxic conditions found in the microenvironment of solid tumors, the uptake of the Gd@C₈₂(OH)₂₂ nanoparticles by MDA-MB-231 cells was increased and this ability of the nanoparticles to accumulate in areas of tumor hypoxia might act in concert with the enhanced permeability and retention effect to promote tumor targeting [1]. Gd@C₈₂(OH)₂₂ nanoparticles also eliminated CSC populations of MDA-MB-231 cells grown under hypoxic conditions. Finally, the authors reported no appreciable systemic toxicity in mice following ip. injection of Gd@C₈₂(OH)₂₂ once daily for 21 days. In sum, Gd@C₈₂(OH)₂₂ nanoparticles were shown for the first time to act as CSC inhibitors in a mouse model of breast cancer while sparing normal tissues. Further research to unearth the underlying mechanism(s) along with studies to monitor the long-term safety is warranted and may promote the clinical translation of these novel nanomedicines.

Targeted nanoparticles for the resolution of inflammation

Evaluation of: Fredman G, Kamaly N, Spolitu S et al. Targeted nanoparticles containing the proresolving peptide Ac2–26 protect against advanced atherosclerosis in hypercholesterolemic mice. Sci. Transl. Med. 7(275), 275ra20 (2015).

Cardiovascular disease is a leading cause of mortality and morbidity worldwide. The primary cause for cardiovascular disease is atherosclerosis, a chronic inflammation of the arterial wall that results in the formation of so-called plaques, focal accumulations of lipids, smooth muscle cells and foam cells (i.e., fat-laden macrophages). The normal inflammatory process is followed by a resolution phase that promotes tissue repair. However, in atherosclerosis, the resolution phase appears to be defective. In a recent study, Fredman et al. [3] encapsulated a small fragment of annexin A1 – a protein known to promote the resolution of inflammation – in polymeric nanoparticles to target advanced atherosclerotic lesions or plaques. Previous animal studies have shown that the Ac2–26 peptide is rapidly cleared from plasma, necessitating repeated administrations of the peptide, and this precludes the use of the peptide alone in a chronic disease such as atherosclerosis. For this reason, the authors encapsulated the Ac2–26 peptides in nanoparticles composed of poly(lactic-co-glycolicacid) (PLGA) and poly(ethylene glycol) (PEG) designed to slowly release the peptides and targeted the nanoparticles (NPs) to the lesions through the addition of a collagen IV-binding peptide. The authors found that the Col IV–Ac2–26–NPs homed to atherosclerotic lesions in a mouse model following intravenous (iv.) injection. One hallmark of advanced atherosclerotic plaque progression is thinning of the protective layer or cap of subendothelial collagen that overlies the necrotic core of the lesion. The authors found that iv. administration of the Col IV–Ac2–26–NPs once daily for 5 weeks increased the subendothelial collagen in aortic root lesions in mice with advanced atherosclerosis. This was associated with a decrease in lesional collagenase activity. Notably, the amount of collagen in the liver and spleen was unaffected by Col IV–Ac2–26–NPs, suggesting that the increase in the aortic root was a targeted process not associated with potentially adverse fibrosis in other organs. However, the mechanism(s) of these effects and their importance for fibrous cap remodeling in atherosclerosis remain to be elucidated [3]. Additionally, the Col IV–Ac2–26–NPs suppressed generation of reactive oxygen species and increased the production of the proresolving cytokine, IL-10. Ac2–26 targets the N-formyl peptide receptor, FPR2 and Fredman et al. could show that the protective effects of the Col IV–Ac2–26–NPs were mediated in an FPR2-dependent manner as the response to Col IV–Ac2–26–NP treatment was blunted in atherosclerotic mice deficient for FPR2 in myeloid cells. The study of Fredman et al. exemplifies how a nanomedical approach may be harnessed for the treatment of atherosclerosis based on the promotion of the resolution phase of inflammation as opposed to direct inhibition of inflammation, which might impair the host defense in this chronic disease. In a related study [4], the Col IV–Ac2–26–NPs were shown to accelerate epithelial barrier repair during the resolution phase of experimental colitis or inflammation of the colon in mice. Together, these studies suggest that nanoparticles may be used for delivery of proresolving mediators.

Nanoparticles for tissue depth-independent phototherapy

Evaluation of: Kotagiri N, Sudlow GP, Akers WJ, Achilefu S. Breaking the depth dependency of phototherapy with Cerenkov radiation and low-radiance-responsive nanophotosensitizers. Nat. Nanotechnol. 10(4):370–379 (2015).

The combination of light and photosensitizers for phototherapeutic interventions such as photodynamic therapy is recognized as a treatment modality that is both minimally invasive and minimally toxic. However, conventional phototherapy is limited by the shallow penetration of light into tissues and the reliance of tissue oxygenation to generate cytotoxic oxygen radicals. In a recent study, Kotagiri et al. [5] have reported a way to overcome the depth limitation of photodynamic therapy by using Cerenkov radiation (CR) from radionuclides as a light source to activate nanoparticles that act as oxygen-independent photosensitizers. CR, identified in the 1930s by Pavel Cerenkov who later won the Nobel Prize in Physics for the discovery, occurs when charged particles, such as positrons and electrons, travel faster than the speed of light in a given medium. Radiolabeled ¹⁸F-fluorodeoxyglucose (FDG) is widely used in PET and serves as an excellent source of CR. The authors hypothesized that FDG could be used in combination with TiO₂ nanoparticles for CR-induced therapy (CRIT) to destroy cancer cells. TiO₂ nanoparticles produce oxygen radicals when exposed to light, independently of the presence of oxygen. To promote the homing of the nanoparticles to tumor cells in vivo, the authors functionalized the nanoparticles with transferrin; however, it could not be clarified whether the transferrin ligand afforded higher tumor uptake in mice or whether this resulted from the enhanced permeability and retention effect [5]. To further enhance the efficacy of CRIT, the authors also incorporated titanocene into the nanoparticles; titanocene has previously been studied in clinical trials as a chemotherapeutic drug, but the present work provided evidence that it may act as a phototherapeutic drug instead. Notably, the photofragmentation of titanocene generated a different class of cytotoxic oxygen radicals that could complement those generated by TiO₂ nanoparticles. Using HT1080 fibrosarcoma cells, Kotagiri et al. evaluated the cellular internalization of the nanoparticles and in vitro CRIT-mediated toxicity. They found that internalization of both the nanoparticles and the radionuclide was required for CRIT. Next, the authors administered nanoparticles in combination with FDG as a light source in mice with aggressive HT1080 tumors. Following validation of the tumor-selective uptake, they administrated the different nanoparticle plus FDG formulations, i.e., FDG/TiO₂-Tf NPs, FDG/Tf-Tc NPs, and FDG/TiO₂-Tf-Tc NPs using a single dose (1 mg kg^-1 bodyweight). When injected into the bloodstream with FDG, TiO₂-Tf-Tc NPs showed the most significant effect. Fifteen days after treatment, tumors in treated mice were eight-times smaller than those in untreated mice, and survival increased to 50 days, compared with 30 days for both TiO₂-Tf and Tf-Tc NPs. A similar pattern of inhibition of tumor growth was evidenced in mice with a sc. A549 lung cancer xenograft. Histological analysis did not reveal significant toxicity in the liver and kidneys after CRIT indicating the absence of systemic toxicity. Overall, this ingenious approach opens up the possibility of treating a variety of lesions in a depth- and oxygen-independent manner.