We have explored the use of mesoporous silica nanoparticles (MSN) for enhanced loading and superior delivery of anti-cancer drugs. This have been motivated because of some unique features of MSN, namely, high specific surface area, large internal pore volume, regularly arranged pores with a fixed diameter, ability to chemically functionalize the external surface of MSN and its good biocompatibility. Mesopores (of diameter > 2 nm) present in MSN provide adequate space to incorporate drug molecules inside them and protect the drug from degradation by body enzymes, before reaching to the cancer-site. However, there are two key challenges in the delivery of any anticancer drug. Firstly, to achieve much higher release of drug at lower pH (5.5) of cancer cells, in comparison to physiological pH (7.4) of healthy cells, and secondly, to reduce the initial burst-release of the drug from MSN, in order to have a controlled release over a long period of time. To address these issues, firstly, MSN of external particle diameter between 42 to 64 nm was synthesized by using TMMS as a growth-quencher during synthesis, in order to effectively use enhanced permeability and retention (EPR) effect.
In addition to this, to address the first challenge of pH-specific release in particular, a spectrum of different MSN samples with unimodal, internal pore diameters ranging from 2.5 nm to 5.2 nm were prepared, using either hexane or decane as a pore expander. This helped us to identify the highest possible drug loading and release. To that end, the anticancer drug gemcitabine was loaded into the pores of MSN. Highest gemcitabine loading of 14.92% could be achieved with the optimum MSN sample having internal pore diameter of 5.2 nm. This sample, at the lower pH of 5.5, gave the highest cumulative gemcitabine release (over a period of 7 days) of 58%, in comparison to only 22% at the physiological pH of 7.4. This implies, we could achieve a reasonably high differential-drug release (between the lower and higher pH) of 36%. To address the second challenge of controlled release, subsequently, MSN with this particular 5.2 nm internal pore diameter was coated with pH sensitive polymers, like either chitosan or poly (D, L-lactide-co-glycolide) (PLGA), both of which degrade at pH less than 6. Use of chitosan coated MSN resulted in lowering of the release at physiological pH, from 22% down to only 5%, thereby increasing the differential-drug release (between the lower and higher pH), from 36% to an even higher value of 53%. Finally, conjugation of transferrin (Tf) on PLGA coated MSN reduced the initial burst release (within the first 24 h period) to only about 20%, compared to as high as 50% release (in 24 h period) in absence of the coating. This ensured that most of the remaining loaded drug was slowly released over the total period of 7 days, sustaining a desired concentration of the drug throughout the dosage period.
The above best sample (i. e., MSN with 5.2 nm internal pore diameter and loaded with gemcitabine drug, whose external surface was coated with PLGA and conjugated with transferrin), along with some control samples were used for cytotoxicity studies [by sulforhodamine B (SRB) assay], against pancreatic cancer cell lines (MIA Paca-2). Overall, we find that, drug loaded MSN have better cytotoxic effect compared to free gemcitabine drug, without impregnation in MSN. For example, uncoated MSN (containing impregnated drug) could inhibit growth of only 50% cells, without any cell-killing. However, either chitosan or PLGA coated MSN (with impregnated drug) could achieve killing of 50% of cells, in addition to inhibition of the rest 50%, since premature drug release (probably outside the cells) was prevented by the polymer. Furthermore, conjugation of Tf ligand on the PLGA coated MSN (the optimized, best sample) showed the most promising result, with a maximum of up to 60% cell killing, as nanoparticles were better uptaken by MIA Paca-2 cells, through ligand–receptor interactions. In comparison to all these samples, gemcitabine without any MSN have not shown any cytotoxicity effect on MIA Paca-2, which proves that MSN is an effective drug-carrier compared to the free drug. Thus, combination of selective particle and pore diameter, alongwith polymer coating and ligand conjugation could achieve enhanced cancer-cell killing in a selective and targeted manner (compared to healthy cells), with a longer drug-release period.