The work in this paper highlights how complexation of N-BPs with the RALA peptide delivery platform, augments the anticancer effects in GBM. LN229 and T98G cells treated with RALA/ALN NPs exhibited a significant decrease in the cell viability in vitro, as demonstrated by the apoptosis and clonogenic assay. These NPs are evidence of a promising novel therapeutic in the treatment of GBM.
The complexation of anionic compounds with the RALA peptide is dictated by electrostatic interactions.17 BPs are distinguished by the unique ‘P-C-P’ structure where anionic phosphonate groups are at the core of the parent structure.9 At pH 7.4, BPs such as ALN and risedronate (pKa 6.3 and 5.25, respectively) are deprotonated thereby facilitating electrostatic interactions with RALA. For example, Nancollas et al. found that at a higher pH (pH 7.4) more phosphonate protons dissociated to give rise to a negative BP species,31 which is advantageous for NP formulation; N-BPs were dissolved in TE buffer (pH 8.0) prior to NP formulation to increase the affinity for cationic RALA. ALN was stable in TE buffer over a 7-day period where the pH remained steady at pH 7–8. TE buffer was deemed biologically appropriate for formulation as it is commonly used for the preservation of DNA during storage, with no associated adverse effects observed in cells.32 The composition of 1% Tris with synthetic amino acid EDTA maintained the solution at an alkaline pH. EDTA is a hexadentate ligand which sequesters ions such as Ca2+ and Mg2+. After being bound by EDTA into a metal complex, metal ions remain in solution but exhibit diminished reactivity.33 Therefore it is possible that the inhibition of surrounding cationic ions within an aqueous solution strengthens the electrostatic interactions between RALA and N-BPs thus stabilising the peptide-NP complex, a method that could be employed for future formulations.
RALA/ALN NPs < 100 nm in size with an overall positive surface charge were formed with a zeta potential of + 16 mV or higher across all mass ratios. The PDI remained less than 0.7 with no significant difference observed at each individual mass ratio. Additionally, there was no significant change in particle size over a range of temperatures up to 40oC and over 28 days, exhibiting the highly stable nature of the RALA/ALN NPs. Cationic NPs are advantageous for cell uptake, with particles binding strongly to lipid bilayers, triggering endocytosis. Indeed, Oh and Park found that positively charged cysteamine-gold NPs, up to 40 nm in size, were internalized at a higher rate compared to negatively charged or zwitterionic gold NPs in monocytes and macrophages.34
Given that ALN lacks a chromophoric group, AF647-RIS was used as a fluorescent N-BP analogue to quantify RALA complexation and cellular uptake of N-BPs. The presence of a bulky AlexaFluor®647 fluorescent group did not prevent RALA complexation. There was no significant increase in the mean hydrodynamic size as particles remained below 100 nm. However, the zeta potential significantly increased up to approx. +24 mV. It is not possible to accurately predict how binding between the cationic peptide and varying moieties will occur due to the spontaneous nature of self-assembly, however, it is possible that due to the higher MW of AF647-RIS, additional RALA molecules concentrated at the particle surface for efficient complexation. It was shown that RALA effectively complexed fluorescently labelled AF647-RIS, providing an encapsulation efficiency > 95% at a mass ratio of 10:1, indicating highly efficient condensation.
Characteristically, BPs are taken up through fluid-phase endocytosis, which is an actin-dependent endocytic pathway where ruffling plasma membranes fuse to enclose fluid.5,35,36 It is thought that during osteoclastic bone resorption, acidification of the vesicle leads to neutralisation of the phosphonate head groups, which in turn, facilitates diffusion across the endosomal membrane. However, this process results in the entrapment of the BP in the acidic interior of endosome vesicles so only a small amount is able to exert an effect within the cytosol.5 Coxon et al. found the internalisation of N-BPs was higher in resorbing osteoclasts compared to non-resorbing osteoclasts, calvarial osteoblasts and MCF-7 breast cancer cells (those not associated with a resorption pit).37 Therefore, it is highly unlikely that BPs are able to be effectively taken up by non-resorbing, non-osseous cells to reach the concentrations required for full efficacy. RALA is particularly efficient in entering cells and escaping the endosome. RALA complexation could potentially reduce bone-targeting, leading to enhanced cellular uptake in non-osseous cell types through increased BP bioavailability. The uptake of AF647-RIS was observed across all mass ratios in both T98G and LN229 cell lines. Notably, the highest uptake was in LN229 cells which was confirmed through confocal microscopy. Inhibition of FPPS enzymatic activity occurs within the intracellular compartment and it is here BPs exert a therapeutic response. Previous studies have proven RALA is capable of effectively escaping entrapment within the late endosome promoting delivery of anionic cargo to the site of action.5,16−22
In this study RALA formulated BPs exert anticancer effects in a dose-dependent manner with both fresh and lyophilised NPs. A potentiation factor of 14.6 and 13.4 was achieved in both LN229 and T98G GBM cell lines, respectively, when cells were treated with RALA/ALN NPs compared to free drug alone. Dose response studies showed particles retained functionality post-lyophilisation where the cytotoxicity of ALN was potentiated through RALA complexation and notably demonstrated more potent effects on cell viability. The greatest potentiation was observed in LN229 cells compared to T98G cells at a factor of > 35.1 and > 14.2, respectively, when treated with lyophilised NPs. Lyophilisation is essential if these NPs were to become a product as electrostatic NPs are destabilised in saline solution due to the presence of NaCl molecules. Sugars such as trehalose are advantageous as they do not impact upon charge and, in fact, create a product that upon re-constitution becomes isotonic. The optimal trehalose concentration was determined with DLS and TEM images indicating that particles retained a uniform spherical nanosized structure post-lyophilisation and notably lead to a lower PDI.
Dual staining with PI and FITC-Annexin V was used to evaluate the apoptotic effects of RALA/ALN NPs on LN229 and T98G cells. Results were aligned with the IC50 dose after 72 h when cells exhibited close to equal populations of live and late apoptotic/necrotic cells. Differences in apoptosis could be attributed to gene expression between cell lines. Hong et al. reported that LN229 and U251n GBM cells expressed multiple stem cell markers such as Nestin, Sox2, Musashi-1 and CD44 with evidence of higher migration and colony formation potential compared to T98G and U87 cells, which did not express Nestin, Sox2 and Musashi-1.38 Furthermore, the fast-growing nature demonstrated by LN229 cells, could correlate to a higher sensitivity to cytotoxic therapies. For example, Wang et al. evaluated the expression levels of selected markers in a panel of GBM cell lines and determined the sensitivities of GBM cell lines in response to TMZ. The authors found U87 and LN229 were more sensitive to TMZ treatment than T98G and LN18 and additionally, the expression levels of the genes ABCC3, TNFRSF1A, and MGMT were higher in T98G and LN18 than those in U87 and LN229. Notably ABCC3 is part of the ABC subfamily of multidrug resistance proteins, known to be a major detriment to cancer therapy.39,38 ABCC3 is an organic anion transporter which facilitates the efflux of a range of conjugated and unconjugated organic anions, as well as a range of therapeutic agents such methotrexate.40 Therefore, it is possible that efflux of the negatively charged ALN, could be modulated by ABCC3 in T98G cells, resulting in reduced potency of RALA/ALN NPs in this particular cell line. Further studies ABCC3 knockdown studies could confirm this.
RALA/ALN NPs proved effective in the inhibition of cell growth, cell survival and induction of apoptosis in vitro. However, it was necessary to determine the mechanism by which this occurred. For example, Ras mutations are found in approximately 30% of all cancers with some cancers having much higher mutation rates.41 These mutations are a negative predictor for a number of therapies, therefore disruption of Ras membrane localisation presents a potential anticancer strategy.42 These findings indicate that H-Ras prenylation was significantly inhibited in all cell lines when treated with RALA/ALN NPs, indicating one likely mode of action. In a similar study, Lühe et al. reported an inhibition of FPPS occurred 1 h after N-BP treatment in vitro followed by decreased levels of prenylated proteins at 24–48 h and increased cytotoxicity post 48 h, findings comparable to ours.43 This suggests protein prenylation must be reduced below a critical level before the cytotoxic effects occur typically 72 h post-treatment. Indeed, abnormal protein prenylation has been attributed in the progression of numerous cancer types including prenylation of Ras and Rho which has been implicated in gliomas.44 However, it would be prudent to conduct further studies where other small GTPase variants are utilised to elucidate the full extent of the anticancer effects of RALA/ALN NPs in GBM. Furthermore, application of RALA/ALN NPs could be broadened to include other cancer types where Ras mutations are predominant (i.e. pancreatic cancer).45
Subsequently, spheroid models were established to simulate solid tumour conditions in vitro using RALA NPs for the first time. Impressively, a significant decrease in spheroid growth and doubling time was observed upon treatment with RALA/ALN NPs in both LN229 and T98G cell lines, highlighting the potential antitumour effects in vivo. Lagies et al. found that 3D spheroid cultures shared strikingly metabolic similarities to the organ tissue such that authors suggested optimised 3D culture techniques could possibly replace animal testing.46 Furthermore, Di Liello et al. cultured 3D spheroids from patient derived non-small cell lung cancer tumours for ex-vivo analysis.47 Authors reported that drug sensitivities to cisplatin-based chemotherapy and anti-programmed death 1 drug sensitivity were consistent from the patient clinical response to 3D culture. The results highlight how 3D cultures enable the formulation of highly translatable therapies for clinical use and demonstrate RALA/ALN NPs could significantly reduce tumour burden in vivo.
Currently, BPs do not reach significant accumulation levels in the brain after intravenous administration due to a high predisposition to distribute in skeletal regions.48 Systemic administration of BP would require the use of a high quantities, exceeding the clinical dosing regimens given to patients to achieve an effective concentration in the brain. Therefore, delivery of BPs complexed with the RALA peptide holds clinical relevance as it could lead to a reduction in the therapeutic dose required as indicated by the significant potentiation effect. Taken together, this study indicates that the RALA/ALN NPs could be a nano-formulation to deliver ALN, an FDA-approved prenylation inhibitor, into GBM tumour tissues for enhanced anti-tumour effects.