In vitro cell-based studies (I-IV)

4.6.1 Cell lines and cell culture (I-IV)

Human colon adenocarcinoma Caco-2, mucus-secreting intestinal cells HT29, and macrophage cells RAW 264.7 were obtained from American Type Culture Collection (USA); and the human prostate cancer cells PC3MM2 were kindly provided by Prof. Akseli Hemminki (Faculty of Medicine, Biomedicum Helsinki, University of Helsinki, Finland).

All the cell lines were cultured in Dulbecco’s Modified Eagle’s Medium (DMEM) supplemented with 10% FBS, 1% non-essential amino acids, 1% L-glutamine, penicillin (100 IU/mL) and streptomycin (100 mg/ml). The cell cultures were kept in a standard incubator (16 BB gas, Heraeus Instruments GmbH, Germany) at 37 °C in an atmosphere of 5% CO2and 95% relative humidity. The growth media was changed every other day for all the cell lines until the day of the experiments, and the subculturing performed at 80%

confluency using trypsinPBSEDTA solution.

4.6.2 Cytotoxicity (IIV)

The in vitro cytotoxicity of the particles (I and II) and the drug loaded particles (III and IV) was assessed by an ATP-based cell viability kit, according to the manufacturer’s specifications. Briefly, the cells were seeded in 96-well plates (Corning Inc., USA) at a concentration of 5×10⁵HT-29 cells/mL, 2.5×10⁵Caco-2 cells/mL, and 1×10⁵ PC3MM2 cells/mL; and allowed to attach overnight. Next, the medium was replaced with different concentrations of particles dispersed in 1× HBSS‒HEPES (I, II and IV) or DMEM enriched with 10% FBS (III), to allow the release of SFN. A positive (1% Triton X-100 solution) and negative control (HBSS–HEPES buffer solution) were included in each 96-well plate.

After predetermined incubation times, the NPs were removed from the wells and washed with fresh HBSS–HEPES buffer. Then, 100 μL of CellTiter-Glo^®reagent assay (Promega Corporation, USA) were added to each well and the luminescence analyzed with a Varioskan Flash Multimode Reader (Thermo Fisher Scientific, USA). All the experiments were performed at least in triplicate (n ≥ 3).

4.6.3 Cell‒particle interactions and intracellular trafficking (III and IV)

4.6.3.1 Flow cytometry analysis (III and IV)

The internalization of NPs (III and IV) by RAW 264.7 (III), PC3MM2 (IV), and EA.hy926 cells (IV) was studied using flow cytometry. The cells were seeded in 6 well-plates (2 mL) at a concentration of 2.5×10⁵ cells/mL, and incubated overnight at 37 °C to allow the attachment of the cells to the well. Next, the medium was removed from the wells and washed once with HBSS‒HEPES buffer solution. About 1 mL of different concentrations of Alexa Fluor 488 (AF 488) (III) and Alexa Fluor 647 (AF 647) (IV)-labeled NPs dispersed in DMEM supplemented with 10% FBS (III) or HBSS‒HEPES buffer solution (IV) were added to each well. In each experiment, a negative control of cells without treatment, incubated either with DMEM 10% FBS or HBSS‒HEPES was used for comparison.

All the samples were incubated for 3 h at 37 °C, whereupon the medium was removed from the wells and washed three times with HBSS‒HEPES to remove any free NPs not associated with the cells. Finally, the cells were harvested by incubating them with tryspsinPBS ethylenediaminetetraacetic acid (trypsin‒PBS‒EDTA) for 5 min at room temperature, after which the cells were centrifuged for 5 min at 900 rpm, and the extracellular fluorescence of the AF 488 NPs (III) quenched with trypan blue. Then, the cells were washed and the medium replaced with 300 μL of PBS with EDTA (5 mM). NP‒

cell internalization and association were quantified by flow cytometry (Galios Flow Cytometer; Beckman Coulter, Inc.; laser 642 nm) and the data was post-processed using the software FloJo X (FlowJo, LLC).

4.6.3.2 Confocal fluorescence microscopy analysis (III and IV)

The NP’s intracellular localization (III and IV), lysosomal escape (IV), and DOX intracellular release (IV), were studied by confocal fluorescence microscopy with RAW 264.6 (III) and PC3MM2 cells (IV). For this, the cells were seeded in an 8-chamber slide (Nunc Lab-Tek II Chamber Slide System, Thermo scientific, Inc., USA) at a concentration of 2.5×10⁵ cells/mL (200 μL/chamber), and incubated overnight at 37 °C to allow the attachment of the cells. Next, the medium was removed from the chamber and replaced with 200 μL of a 100 μg/mL suspension of FITC-labeled (III) and AF 647-labeled (IV) NPs in DMEM supplemeted with 10% FBS (III) or HBSS‒HEPES buffer solution (IV). A negative control of the cells without treatment, incubated with either DMEM containing 10% FBS or HBSS‒HEPES buffer solution was included in each chamber.

The NPs were then incubated with the cells, whereupon the NP suspensions were removed and the NPs not associated with the cells washed out with HBSS‒HEPES buffer solution. Then, the different cell components were stained according to the needs of each study. For RAW 264.7 cells (III), the cell membrane was stained with CellMask Deep Red (Life Technologies, USA), according to the manufacturer’s instructions. In the case of PC3MM2 (IV), either the lysosomes to study the lysosomal escape or the nuclei to study intracellular release of DOX, were stained. The lysosomes were stained by incubating the cells with LysoTracker Red for 30 min at 37 °C. The nuclei were stained with DAPI after fixation of the cell culture, by incubating the cells with 300 μM DAPI in 1× PBS for 3 min.

All the cell cultures were fixed with 2.5% glutaraldehyde solution in HBSS‒HEPES buffer solution for 30 min at room temperature followed by five times washing with HBSS‒

HEPES buffer solution. PBS was finally added to the chambers and then the cells were visualized with a confocal fluorescence microscope (Leica inverted confocal microscope SP5 II HCS A) using HeNe (633 nm), DPSS (561 nm), Ar (488 nm), and UV-diode (405 nm) as the laser sources, and a water immersion objective HCX PL APO 63×.

4.7 Hemotoxicity (IV)

RBCs were purified from human blood extracted from anonymous donors at the Finnish Red Cross Blood Service. Blood was stabilized with heparin and used within 2 h.

For the isolation of RBCs, 5 mL of the blood sample were mixed with 10 mL of sterile Dulbecco’s PBS pH 7.4 (D-PBS) and centrifuged for 6 min at 3000 rpm in order to separate the RBCs from the serum. RBCs were then washed five times, and finally diluted twenty times with D-PBS. About 60 μL aliquots of this suspension were added to 240 μL of the IO@PNP and IO@iRGD-PNP to a final concentration of 25, 50, 100, and 200 μg/mL. The suspension was mixed and incubated for 1, 4, 6, and 24 h. Subsequently, at each time point, the sample was homogenized and 50 μL were transferred to an Eppendorf. The sample was then centrifuged for 3 min at 13000 rpm and the supernatant transferred to a 96-well plate to measure the absorbance intensity of hemoglobin at 577 nm using a microplate reader. D-PBS and water were used as negative and positive controls, respectively.

For the timepoint corresponding to 4 h, the morphology of the RBCs was also investigated by SEM. For this, the pellet of RBCs was re-dispersed in 2.5% glutaraldehyde

and incubated for 1.5 h in order to fix the membrane of the cells. After the incubation time, the RBCs were washed three times with D-PBS and post-fixed in 1% osmium tetraoxide in 0.1 M of sodium cacodylate buffer (pH 7.4) for 1 h. The cells were then dehydrated in increasing concentrations of 50, 70, 96, and 100% of ethanol for 5, 10, 20, and 15 min, respectively [278]. Finally, the cell’s suspensions were dropped onto plastic coverslips, dried, and sputter coated with platinum prior to SEM imaging.