Characterization of the nano systems
3.1.1. Elemental analysis
The results of the elemental analysis conducted to ensure binding of drugs to the carbon nanotubes can be seen in Table 4. Naked carbon nanotubes does not contain any nitrogen, but detectable amounts were seen in drug-functionalized nanotubes, indicating effective binding of the drugs to the carbon nanotubes. Considering the mass content of azote (N) in 100 g of MWCNT, a loading of 0.6 mmol INH/g can be inferred for MWCNT-INH, and 1.4 mmol FLX/g for MWCNT-FLX. Because of minor CNT mass reduction due to centrifugation and washing, the carbon balance could not be used to estimate the loading.
Comparison of infrared spectroscopy of non-functionalized nanotubes with drug-activated nanotubes
The Fourier Transform Infrared (FTIR) spectrums of carboxylated and drug-functionalized nanotubes are shown in Figures 1-3. The peak at ~1525 cm-1 corresponds to the C=C double bonds from the nanotube wall. Also, the peaks at 3304 cm−1 (Figure 1) 3437 cm−1 (Figure 2) and 3433 cm−1 (Figure 3) correspond to the carboxylic acid group. The appearance of new peaks with lower wave number at ~1678 cm−1 in figure 2 and ~1630 cm−1 in figure 3 were assigned to amide bond. So The FTIR spectrum of MWCNTs-INH (Figure 2) and MWCNTs-FLX (Figure 3) confirmed the formation of amide groups on the MWCNTs surface.
Scanning electron microscopic results
Evidence for the functionalization of multi-walled carbon nanotubes can be obtained by SEM electron microscopy images. Figure 4 shows the SEM images of MWCNTs, MWCNTs-INH and MWCNTs-FLX. As visible on the figures, the drug-functionalized nanotubes have a different morphology and appear rougher compared to the naked nanotubes, which confirms that MWCNTs were functionalized with drugs.
Results of different types of treatment with INH, FLX, MWCNTs-INH and MWCNTs-FLX
The effective concentration which could be considered as MIC value for each treatment are summarized in table 5. The results obtained by bacterial growth observation for all treatment groups for H37Rv, MDR, and XDR strains is summarized below.
H37Rv treatments with the different doses of INH, FLX, INH+ FLX, MWCNTs-INH, MWCNTs-FLX and MWCNTs- INH+ MWCNTs- FLX
After incubation with INH for two weeks, a significant decrease in bacterial growth was observed from a dose of 1/1024 (0.14 µg/mL) and no growth was observed from dilutions of 1/512 (0.28 µg/mL) (Figure S1). Also, a significant reduction in growth was observed at a dilution of 1/64 from MWCNTs-INH (1.56 µg/mL containing 0.13 µg/mL INH) and no growth was observed at a dilution of 1/32 (3.125 µg/mL containing 0.26 µg/mL INH) (Figure S2). The efficacy results for FLX indicate that bacterial growth is stopped at dilutions of 40, (1.6 mg/mL) (Figure S3). The treatment results of MWCNTs-FLX showed that no growth was observed from dilution of 2 (222 µg/mL, containing 96 µg/mL FLX) (Figure S4). For Checkerboard treatments on H37Rv, the different concentrations of both drugs from dilution of 2 to 1/4 for MWCNT-FLX and 1/64 to 1/512 of MWCNT-INH were tested. The bacterial growth was significantly reduced when the two nano drug systems were combined, with dilutions of 1/128 (0.78 µg/mL containing 0.067 µg/mL of INH) of MWCNTs-INH sufficient to prevent bacterial growth when used in combination with dilution of 1/2 (55.5 µg/mL containing 24 µg/mL of FLX) of MWCNTs- FLX (Figure S5). Checkerboard treatment on H37Rvwith FLX and INH was carried out in a previous study and results showed that dilutions of 1/256 of INH (0.56 µg/mL) combined with dilution 20 of fluoxetine (0.8 mg/mL) inhibited bacterial growth. The antibacterial effect of MWCNT alone on H37Rv was observed at the concentration 228 µg / mL (Figure S6).
MDR & XDR treatments with the different doses of INH, FLX, INH+ FLX, MWCNTs-INH, MWCNTs-FLX and MWCNTs-INH + MWCNTs-FLX
For the MDR strain, INH at dilution of 1/128 (1.12 µg/mL) was required to observe a significant effect on bacterial growth (Figure S7). Similarly for the XDR strain, INH at dilution of 1/128 (1.12 µg/mL) had significant effect on bacterial growth (Figure S8). Besides, growth of the MDR strain was observed at the dose of 3.2 mg/mL and only has few growth reduction at 1.6 mg/mL for FLX (Figure S9), while for the XDR strain only a slight growth reduction was observed at this dose of FLX (Figure S10). Results showed that bacterial growth of MDR and XDR strains were been inhibited at the dilution of 1/16 (6.25 µg/mL, containing 0.52 µg/mL of INH) and 1/8 (12.5 µg/mL, containing 1.04 µg/mL of INH) of the MWCNTs-INH respectively (Figures S11, S12).
MWCNTs-FLX has few antibacterial effect on MDR at dilution of 1/2 (55.5 µg/mL, containing 24 µg/mL of FLX) (Figure S13) and also on XDR at the dilution of 1/16 (6.9 µg/mL, containing 3.5 µg/mL of FLX) (Figure S14).
Checkerboard treatment results of MDR and XDR strains with INH and FLX showed that, in the treatment with dilution of FLX 20 (800 µg/mL) in combination with isoniazid in the dilution of 1/256 (0.56 µg/mL), the bacterial growth were stopped (Figure S15). Besides, in XDR treatment group at the dose of FLX 40 (1.6 mg/mL) in combination with INH at a dose of 1/128 (1.12 µg/mL) and also at doses of FLX 20 (800 µg/mL) in combination with INH at a dose of 1/256 (0.56 µg/mL), a significant reduction in growth was observed (Figure S16).
After the effect of MWCNTs-INH and MWCNTs-FLX, bacterial growth observation indicated that in the MDR strain, at the dose of 1/2 of MWCNTs-FLX (55.5 µg/mL, containing 24 µg/mL of FLX) in combination with the dose of 1/32 of MWCNTs-INH (3.125 µg/mL, containing 0.26 µg/mL of INH), no growth was observed. In the XDR strain group, the results showed that at doses of 1/16 of MWCNTs-FLX (6.93 µg/mL, containing 3.5 µg/mL of FLX) with a dose of 1/8 of MWCNTs-INH (12.5 µg/mL, containing 1.04 µg/mL of INH), bacterial growth was inhibited (Figure S17).
Quantitative Analyses by Fractional Inhibitory Concentration Index (FICI) for FLX and INH
According to the calculations of FIC index value for all states of action of isoniazid and fluoxetine in both free and conjugated form with carbon nanotubes, it was shown that in all strains, these two drugs have an additive effect on each other both in free and conjugated forms (Table 6).
Gene expression analysis and IL6 and TNFα secretion studies
The data analysis of gene expression in various treatment groups was carried out by the One Way ANOVA method and showed that the expression of both inhA and katG genes in all strains increased significantly in the presence of INH (Figure S18-S20). In the presence of FLX however, both gene expression was unchanged. For all strains tested, every treatment group involving INH saw a sharp increase in inhA expression, irrespective of the presence of FLX or the formulation in MWCNT. In contrast, the expression of katG was significantly reduced by combination therapy with free FLX, and the conjugated form of INH led to weaker levels of expression (Figure S18-S20). Results of the cytokine secretion studies showed that the levels of IL6 and TNFα secretion were significantly increased in all the treated groups, indicating that the drug treatment was able to induce pro-inflammatory pathways in TB-infected macrophages (Figure S21-S23), presumably due to efficient killing of the bacteria. The secretion of IL6 and TNF-α cytokines from TB-infected macrophages were similar between free and MWCNT-conjugated drugs, indicating that the CNT vectorization strategy could maintain an efficient targeting of the intracellular bacteria.