**1. RNA Interference and Challenges in Small Interference RNA Therapeutics**

Discovered in 1986 by the Nobel laureates Fire and Mello [1], RNA interference (RNAi)—also known as post-transcriptional gene silencing (PTGS)—is a compendium of mechanisms involving small RNAs that regulate the expression of genes in a variety of eukaryotic organisms. In simple terms, the RNAi process implies the cleavage of endogenous long double-stranded RNAs into short ribonucleic acid sequences (the so called small interfering RNAs or siRNAs, usually 21–23 bases long) by the action of the Dicer endonuclease [2]. Upon incorporation into the multiprotein RNA-induced silencing complex (RISC), the double helical siRNAs are unwound into two strands: The sense (or passenger) strand is discarded, and the antisense (or guide) strand is paired to a complementary mRNA sequence via the RISC complex. Upon binding, the targeted mRNA is in turn degraded by a RISC subunit (known as Argonaute 2) endowed with endonuclease activity. This last step ultimately results in the prevention of mRNA translation into the corresponding protein or, in other words, in gene silencing. Finally, once the target mRNA degradation is accomplished, the RISC complex can be recycled to digest other targets on the mRNA, greatly improving inhibition efficiency [3].

In the post-genomic era, siRNAs of a synthetic nature can be designed and synthesized under good manufacturing practice (GMP) to target complementary regions on any gene of a known sequence

to achieve its downregulation for curative purposes [4–6]. However, key to the therapeutic utility of these RNAi triggers is the ability to introduce them into their target cells of the human body. Indeed, naked siRNAs are not amenable to therapeutic administration. For instance, when a siRNA is administered intravenously, it is readily digested by nucleases and largely cleared from the kidney glomeruli before reaching the diseased organs. Moreover, the negative charge and large size of a naked siRNA make it difficult to pass through the plasma membrane of a target cell. Even if the siRNA molecules could reach the target tissue and be taken up by target cells, they must avoid degradation in lysosomes via endosomal escape, a process in which the efficiency of these nucleic acid fragments is notoriously low. As a consequence, one of the major challenges in successful RNAi therapeutics is the discovery of safe, efficient and effective siRNA delivery vectors. Such delivery vehicles must at least protect each siRNA from nucleases in the serum or extracellular media, enhance siRNA transport across the cell membrane, and guide the siRNA to its proper location through interactions with the intracellular trafficking machinery. Ideally, both viral and non-viral (nano)vectors can deliver siRNA into cells [7], although, despite impressive transfection efficiency, the use of the former is limited by safety concerns, including genotoxic and immunogenicity-mediated adverse events [8]. On the contrary, non-viral delivery systems have great potential for the safer delivery of siRNA therapeutics, although so far their performance in transfection efficiency has not reached the level requested for full clinical exploitation [7,9–11].

#### **2. Role of Dendrimers as siRNA Nanocarriers**

In the variegated scenario of non-viral (nano)materials for siRNA delivery, dendrimers have quickly grown as a family of synthetic nano-sized, radially symmetric molecules with fine-defined, homogeneous and monodisperse composition endowed with enormous potential as gene therapy nanovectors [12–14]. Structurally, dendrimers are constituted by three distinct domains (Figure 1a): (1) A fundamental atom or, most frequently, a group of atoms defined as the core; (2) the branching units, which, emanating from the core through diverse chemical reactions, allow the dendrimeric molecule to grow in geometrically organized radial layers known as generations (G); and (3) an exponentially increasing number of peripheral surface groups which constitute a multivalent nanoscale array and can therefore form high-affinity interactions with a variety of biological targets [15].

**Figure 1.** (**a**) Cartoon representation of a dendrimer structure highlighting its three, distinct structural motifs: The core (in light blue), the branching units forming the different G generations (in light green), and the terminal groups (in light pink). According to a consolidate dendrimer nomenclature, the core constitutes Generation 0 (G0). Therefore, the subsequent Generations G1, G2, ... Gn refer to the corresponding level of branching, as annotated in panel (**a**). (**b**) Molecular structure of the triethanolamine (TEA)-core poly(amidoamine) dendrimers. For clarity, in panel (**b**), only a dendrimer of Generation 4 (G4) is shown.

From the synthetic viewpoint, both divergent and convergent pathways (or combinations of thereof) can be adopted to prepare dendrimers with different generations with precisely defined, regular structures [16,17]. To date, more than fifty families of dendrimers each with unique chemistry and properties have been produced and are under investigation in a diversity of different biomedical applications [18]. Among these, poly(amidoamine) (aka PAMAM) dendrimers undoubtedly constitute the molecules most widely explored as nanocarriers for both drug and gene delivery [13,19–21]. In the specific field of nucleic acid delivery and release, this popularity can be ascribed to several beneficial features of PAMAMs, including (i) the chemical nature of their terminal groups, which, being primary amines, are fully protonated at the physiological pH of 7.4. This entails extremely favorable electrostatic (Coulombic) interactions of these dendrimers with the negatively charged nucleic acid fragments and the subsequent mutual condensation into nanoscopic particles, often called dendriplexes; and (ii) the presence of tertiary amines within the dendritic branched structure which, becoming protonated at lower pH values pertaining to endosomes and lysosomes, mediate the osmotic swelling and subsequent disruption of the membranes of these vesicles, ultimately promoting the intracellular release of the siRNA cargo. This mechanism, known as the proton-sponge hypothesis, relies on the assumption (under debate; [22]) that the unprotonated amines of PAMAMs can absorb protons as they are pumped into the lysosome/endosome, resulting in more protons being pumped in, thus leading to an increased influx of Cl− ions and water. A combination of the osmotic swelling and a swelling of the dendrimers themselves because of the repulsion between protonated amine groups causes the rupture of the lysosomal/endosomal membranes, resulting in the subsequent release of its contents into the cytoplasm [23].

#### **3. Structurally Flexible PAMAM Dendrimers for Safe, E**ffi**cient and E**ff**ective siRNA Delivery**

Despite the wealth of studies dating back to the early 90s yielding highly promising results for PAMAM-based dendrimers as DNA nanovectors [24–28], only the last 10 years have witnessed systematic investigations of this class of molecules in siRNA delivery (see Table S1 in Supplementary Materials) [19,29–32]. In this arena, successive rounds of structure optimization led us to the design of PAMAM dendrimers with a triethanolamine (TEA) core (Figure 1b) [33]. The rationale behind the conception and synthesis of this new dendrimer family was that the TEA-core molecules, having the branching units starting away from the central amine with a distance of 10 successive bonds, should feature an extended core. As such, they were expected to be less congested in space with respect to the prototypical NH3-core PAMAM dendrimers, in which the branches sprout directly from the small ammonia focal point. As a consequence, the TEA-core dendrimers—with less densely packed branches and terminal units—should be endowed with an enhanced flexibility of their arms and, as such, should perform better as siRNA nanocarriers than their NH3-core counterparts. In essence, the hypothesis of a greater flexibility translating into the more effective enwrapping of the nucleic acid fragment was inspired by the behavior of histones, whose structure dynamics allows for conformational changes related to DNA binding (required for post-translational modification) and unbinding (required to prevent transcription) [34].

#### *3.1. Prediction of Enhanced Flexibility and siRNA Interactions of TEA-Core Dendrimers by Computer Simulations*

The hypothesis of diverse flexibility between TEA- and NH3-core based PAMAMs was verified by atomistic molecular dynamics (MD) simulations [35–37]. Figure 2a,b shows two MD equilibrated structures of the G5 TEA-core and NH3-core PAMAM dendrimers at a physiological pH (7.4), respectively. As intuitively perceived from these images, the TEA-core molecule is characterized by a more open conformation, with a uniform void distribution within its interior, whilst its ammonia-core counterpart is remarkably more compact, featuring a non-homogeneous, restricted void spacing. Thus, the conformation of the TEA-core PAMAM dendrimers is such that the outer branches can freely move and adjust to optimize binding with its siRNA cargo (Figure 2a). On the contrary, the more rigid and

compact structure of NH3-core PAMAMs prevents these molecules from any induced-fit conformational readjustment, and, consequently, not all of the terminal groups are available to self-orient for optimal nucleic acid binding (Figure 2b).

**Figure 2.** Equilibrated molecular dynamics conformations of G5 TEA-core (**a**) and NH3-core (**b**) poly(amidoamine) (PAMAM) dendrimers in physiological solution (pH = 7.4, ionic strength = 0.15 M NaCl). Each dendrimer molecule is represented as colored sticks, some ions and counterions are visualized as purple (Na+) and green (Cl−) spheres, and water molecules are not shown for clarity. Average radial monomer density ρ(*r*) for subsequent generations (from G0 to G5) of the TEA-core (**c**) and the NH3-core PAMAMs (**d**). In all cases, the origin was set at the molecular center of mass. Adapted from [37], published by RSC, 2013.

Quantitative substantiation was obtained from calculating the average radial monomer density ρ(*r*) for each dendrimer type (shown in Figure 2c,d for subsequent dendrimer generations up to G5), a quantity defined as the number of atoms whose centers of mass locate within a spherical shell of radius *r* and thickness Δ*r*. Accordingly, the integration of ρ(*r*) over *r* yields the total number of dendrimer monomers as:

$$N(r) = 4\pi \int\_0^\mathbb{R} r^2 \rho(r) dr. \tag{1}$$

Focusing attention on G5 as an example, in the case of the TEA-core molecule, the whole dendrimer ρ(*r*) curve (shown as a thick continuous line in Figure 2c) is characterized by the presence of two minima—the first (more pronounced) located approximately 10 Å away from the core and the second at around 17 Å—each followed by two relative maxima, at about 13 and 21 Å, respectively. These features of ρ(*r*) constitute a clear indication that the dendrimer core region is denser with respect to the middle–outer molecular portions (which are fairly hollow) and that the higher sub-generation monomers generate a crowded layer at the dendrimer periphery. This, in turn, accounts for the presence of a uniform distribution of hollow spaces in the central dendrimer structure, which can be filled up by a significant number of solvent molecules, as testified by the corresponding thick dashed line in Figure 2c. Considering the same data for the alternative G5 NH3-core dendrimer, the ρ(*r*) profile for the whole molecule (thick continuous line; Figure 2d) is representative of a complete different trend: Indeed, after the core peak, the curve quickly reaches a plateau value that spans the entire central dendrimer region and finally increases again at the molecular periphery. In other words, all dendrimer

sub-generations afford a substantial contribution to the whole density curve, supporting the visual evidence (Figure 2b) of a more uniform monomer distribution within the dendrimeric structure. In line with this observation, the corresponding water density profile (thick dashed line; Figure 2d) does not feature any pronounced maximum in any specific region of the molecule but rather exhibits a uniform distribution throughout the dendrimer interior.

The postulated enhanced ability of the more flexible, extended-core (TEA) PAMAMs in interacting with siRNA molecules with respect to smaller core (NH3) dendrimers was next predicted by computer simulations based on the so-called molecular mechanics/Poisson-Boltzmann surface area (MM/PBSA) methodology [35–40] (see Supporting Information for detailed explanation). To this purpose, the free energy of binding normalized by the total number of charged dendrimer terminal groups (ΔGbind/*N*) between successive generations of the two different PAMAM-based molecules towards the siRNA sequence directed against the mRNA coding for the heat shock protein 27 (Hsp27)—a small molecular chaperone which is a vital regulator of cell survival and a major player in drug resistance—was calculated [35]. This normalization procedure was required to compare the affinity of the different dendrimer generations towards the double-stranded (ds) RNA fragment. As can be seen in Table 1, ΔGbind/*N* is negative for all systems considered, indicating that, under *in silico* physiological conditions (pH 7.4 and 0.15 M NaCl), the association of both dendrimeric nanovectors with their nucleic acid payloads is a thermodynamically favorable and spontaneous process. However, for each dendrimer generation, the TEA-core PAMAMs show a superior affinity for the ds-RNA sequence (i.e., ΔGbind/*N* more negative) with respect to their NH3-core counterparts. Additionally, there is a notable increase in binding strength in passing from G4 to G5, substantially ascribable to an enhanced favorable enthalpic component ΔHbind/*N*. This aspect accounts for the general trend of better binding and, hence, better properties as nanocarriers of high generation dendrimers, which is in agreement with experimental evidence [32] Contextually, the entropic contribution is less unfavorable (i.e., smaller) in the case of the TEA-core molecules. This lower value of –TΔSbind/*N* can be connected again to the enhanced flexibility and, consequently, the greater capacity of the conformational adaptation of all generations of the enlarged-core dendrimers in enwrapping the ds-RNA molecule, followed by an enhanced productive binding of the nucleic acid.

**Table 1.** *In silico* normalized free energy of binding (ΔGbind/*N*) and its major components (binding enthalpy ΔHbind/*N* and entropy variation –TΔSbind/*N*) for G4–G6 TEA-core and NH3-core PAMAMs in complex with heat shock protein 27 (Hsp27) small interfering RNA (siRNA) at pH 7.4 and 0.15 M NaCl. The normalization factor *N* is the generation-specific total number of charged dendrimer terminal groups (*N*). All values are expressed in kcal/mol1. Adapted from [35] with permission of John Wiley and Sons.


<sup>1</sup> Standard deviation for all data in Table 1 is less that 1%.

The equilibrated MD snapshots of these two G5 dendrimer series in complex with the Hsp27 siRNA shown in Figure 3 offer additional insightful structural information. In fact, as can be inferred from Figure 3a, the conformation of the TEA-core dendrimers is such that its outer branches can readily move towards the phosphate backbone of the siRNA during complex formation so that its charged amine groups can arrange themselves via induced-fit for optimal binding with the nucleic acid. On the contrary, the more rigid and compact structure of the alternative PAMAM molecule prevents it from undergoing a significant conformational readjustment required by the induced-fit (Figure 3b); as a consequence, a smaller number of amine groups are available for optimal siRNA binding.

**Figure 3.** Equilibrated molecular dynamics conformations of G5 TEA-core (**a**) and NH3-core (**b**) PAMAM dendrimers in complex with Hsp27 siRNA in physiological solution (pH = 7.4, ionic strength = 0.15 M NaCl). Each dendrimer molecule is represented as colored sticks, some ions and counterions are visualized as light pink (Na+) and light green (Cl−) spheres, the siRNAs are portrayed as light blue ribbons, and water molecules are not shown for clarity. Radial density distribution ρ(*r*) of the dendrimer terminal nitrogen atoms in G5 TEA-core (**c**) and NH3-core (**d**) PAMAMs in complex with Hsp27 siRNA (continuous lines). The corresponding distributions of the siRNA phosphorous atoms in each siRNA/dendrimer complex are shown as dashed lines. Adapted from [35,36] with permission of John Wiley and Sons and Bentham Science Publishers.

This differential behavior in siRNA binding is more evident when analyzing the radial density distributions of the relevant nucleic acid/dendrimer complexes reported in Figure 3c,d. For the G5 TEA-core dendrimer/siRNA complex, the density profiles of the primary, positively charged nitrogen atoms stretches further out towards the molecule periphery due to the electrostatic attraction of the siRNA negatively charged phosphate moieties (Figure 3c). Contextually, the density distribution of the phosphorous siRNA atoms reveals a good penetration of the nucleic acid fragment within the dendrimer outer shell. Contrarily, even in complex with siRNA the NH3-core G5, PAMAM maintains a more compact conformation that is characterized by a high degree of branch back-folding; as a result, the density of the terminal amines on the dendrimer surface is lower, and the corresponding siRNA phosphorous density distribution curve shows only a partial penetration of the nucleic acid within the dendrimer molecular structure (Figure 3d).

#### *3.2. High-Generation TEA-Core PAMAM Dendrimers as E*ff*ective In Vitro and In Vivo siRNA Nanocarriers*

#### 3.2.1. In Vitro Data

The predicted ability of TEA-core PAMAMs to generate nanoscale siRNA/dendrimer complexes (aka dendriplexes) was experimentally verified and characterized in vitro. Figure 4a,b shows the atomic force microscopy (AFM) images of the Hsp27/dendrimer nanoparticles obtained using TEA-core dendrimers from G1 to G7 [41].

**Figure 4.** Three-dimensional atomic force microscopy (AFM) images of (**a**) Hsp27 siRNA in complex with TEA-core PAMAM dendrimers of increasing generation (**G1**–**G7**) and (**b**) a single spherical siRNA/TEA-core dendrimer (**G7)** complex at a final siRNA concentration of 0.0125 mg/L and at a dendrimer-to-siRNA charge ratio (N/P) of 10. (**c**) Gel retardation of Hsp27 siRNA with three different TEA-core PAMAMs ((**G1**) (**a**), (**G4**) (**b**) and (**G7**) (**c**)) as a function of the N/P ratio (from 10/1 to 1/10 from left to right; last lane: Naked siRNA). Adapted from [41] with the permission of the RSC.

While only a few siRNA/dendrimer assemblies can be observed for smaller dendrimers (G1 and G3), an increasing number of nanoscale particles are formed with increasing dendrimer generation (Figure 4); in particular, starting from G4, the siRNA/dendrimer nanocomplexes progressively become more uniform, well-defined, and compact. This suggests that G4 is the lowest threshold TEA-core dendrimer generation for effective siRNA complexation. This assertion is substantiated by the results of the RNA mobility assay illustrated in Figure 4c: While no gel retardation effect is observed with the G1 dendrimer even at the maximum dendrimer-to-siRNA charge (N/P) ratio adopted (10/1), the siRNA mobility is considerably retarded by the G4 molecule for N/P ≥ 2.5, and, for the same N/P value, the siRNA shift in the gels is completely almost prevented by the G7 dendrimer [41]. The complexes formed between siRNA and high generation (≥G4) TEA-core PAMAMs are completely stable in physiological conditions, require strong ionic detergents such as Sodium Dodecyl Sulfate (SDS) to be disrupted and, contrarily to naked siRNA, to show considerable resistance to RNase degradation [33].

The rapid and efficient cellular uptake of siRNA/TEA-core dendrimer nanoassembly was demonstrated using the human castration-resistant prostate cancer (CRPC) PC-3 cell line by confocal fluorescence imaging. In these experiments, a non-silencing (i.e., scrambled) Hsp27 siRNA sequence labeled with the green fluorescent dye Alexa 488 was employed [42]. Figure 5a shows that, 4 h after treatment, cells are populated by the green fluorescent siRNA–dendrimer nanoparticles which reside exclusively in the cell cytoplasm (Figure 5b,c). Contextually, after cell treatment with a similar amount of Alexa 488-labelled naked siRNA, no green fluorescent nanoparticles are detected inside the cells.

The successful siRNA delivery and specific gene silencing of these flexible TEA-core PAMAMs was next validated in the first proof-of-concept (POC) study targeting again Hsp27 PC-3 cells [42]. Prostate cancer is the second most commonly occurring cancer in men and the fourth most commonly occurring cancer overall; although most patients initially respond well to first-line hormone-based therapies associated with androgen ablation, after a very short time period (≈2 years), they unfortunately relapse [43], and no effective therapeutic regimen is available to treat this progressive condition. Therefore, anticancer treatments based on targeting survival genes (e.g., Hsp27) using RNAi constitute an interesting option in contrasting CRPCs [44]. The results from the POC study (Figure 6) show

that transfecting PC-3 cells with TEA-core G7/Hsp27 siRNA complexes yields the potent, specific, and long-lasting downregulation (>50% silencing after five days) of both targeted mRNA and protein [42].

**Figure 5.** (**a**) Confocal fluorescence imaging of G7 TEA-core dendrimer-mediated cellular uptake of siRNA using a non-silencing siRNA sequence labeled with the green fluorescent dye Alexa 488. (**b**) Blue fluorescence images of a nucleus of the same cells stained by TOPRO-3. (**c**) Merged fluorescent images of a and b confirming the exclusive cytoplasm localization of the dendrimer/siRNA complexes. Adapted from [42] with permission of John Wiley and Sons.

**Figure 6.** Quantitative analysis of (**a**) Hsp27 mRNA levels (determined by quantitative reverse transcription polymerase chain reaction (RT-qPCR)), (**b**) Hsp27 protein expression (determined by western blot analysis), (**c**) cell proliferation (determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay), and (**d**) caspase-3/7 activity (measured by a colorimetric assay) in prostate cancer-3 (PC-3) cells three days after the TEA-core G7-mediated delivery of 50 nM Hsp27-targeting siRNA (N/P = 10) (hot pink bars in all panels). Data for dendrimer G7 alone (blue bars), naked Hsp27 siRNA (orange bars), and scrambled siRNA–G7 complexes (gray bars) are shown for comparison. Values are expressed as % relative to control (non-treated cells, green bars). Adapted from [42] with permission of John Wiley and Sons.

Following the knockdown of Hsp27 in PC-3 cells by the G7 TEA-core PAMAM dendrimer/siRNA complex, a remarkable anti-proliferative effect is observed (Figure 6c), in agreement with previous results from Rocchi et al. [45], thus showing that the inhibition of Hsp27 protein expression negatively affects PC-3 cell survival.

The main mechanism beyond this cellular effect was next demonstrated to proceed via caspase-dependent induced apoptosis. Apoptosis is programmed cell death that involves the controlled dismantling of intracellular components while avoiding inflammation and damage to surrounding cells. Apoptotic caspases are a family of endoproteases that provide critical links in cell regulatory networks controlling cell death. Specifically, the activation of these enzymes results in the inactivation/activation of substrates, and the generation of a cascade of signaling events permitting such controlled demolition of cellular components. As seen in Figure 6d, a three-fold increase in caspase-3/7 activity after Hsp27 siRNA/G7 TEA-core dendrimer complex treatment relative to controls is detected, ultimately confirming that Hsp27 siRNA delivered by the G7 TEA-core dendrimer is very effective in inhibiting Hsp27 expression and thereby inducing caspase-dependent anticancer activity in human CRPC PC-3 cells.

The dependence of the TEA-core PAMAM dendrimer-mediated siRNA silencing effect on siRNA concentration, dendrimer generation, N/P ratio and incubation time for transfection was also investigated [42]. A clear dose-dependent gene silencing was observed using the G7 TEA-core PAMAM nanocarrier, in line with a RNAi-mediated gene silencing mechanism. Moreover, in agreement with the molecular simulation results (see Table 1), the gene silencing efficacy was seen to increase with dendrimer generation, the G7 molecule being the best delivery nanovector for Hsp27 siRNA in cell-based assays. With this high generation dendrimer, optimal gene silencing was achieved at an N/P value of 10, this siRNA/dendrimer charge ratio likely providing not only the best nucleic acid degradation protection but also the most effective siRNA deployment into the cellular cytoplasm by virtue of a very efficient endosomal escape mechanism. Finally, an incubation time for transfection of 24 h was determined as the optimal condition for G7 TEA-core PAMAM dendrimer-mediated siRNA delivery to CRPC PC-3 cells for Hsp27 gene silencing.

Sensitivity to serum proteins constitutes an Achilles' heel of cationic nanovectors in siRNA delivery. In fact, since high N/P ratios are always required for efficient nucleic acid transfection (as discussed above), the resulting overall charge of the delivery complexes is positive. As such, electrostatic forces may drive their interaction with the negatively charged serum proteins, this process eventually resulting in the disintegration of the nanoassemblies with consequent premature nucleic acid release and degradation. The lead G7 TEA-core PAMAM dendrimer was indeed not exempt from this drawback, as no Hsp27 gene silencing was observed in the presence of 10% serum with G7 nanovectors loaded with 50 nM Hsp27 siRNA at the optimal, serum-free N/P ratio of 10. In trying to overcome these negative results, gene silencing experiments were performed at progressively increasing dendrimer-to-siRNA ratios. Potent Hsp27 gene silencing is indeed reached in PC-3 cells at N/P = 40, supported by a strong inhibition of cell proliferation and high caspase-3/7 activation, as shown in Figure 7. Pleasingly, the potent in vitro anticancer activity of the G7 TEA-core PAMAM-based nanovectors was paralleled by the complete absence of toxicity, as assayed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) tests for cell viability and by lactate dehydrogenase (LDH) release for cell membrane damage [42]. Finally, no acute toxicity was observed in preliminary tests conducted by treating healthy mice via a tail vein injection with scrambled siRNA–G7 complexes, naked Hsp27 siRNA, and the G7 TEA-core dendrimer alone, as well as a glucose solution as control.

In aggregate, these findings supported the concept that the flexible, high-generation TEA-core PAMAM dendrimers could be effective nanovectors for in vitro siRNA delivery.

*Pharmaceutics* **2019**, *11*, 351

**Figure 7.** (**a**) Effect of N/P ratio on Hsp27 gene silencing, (**b**) inhibition of cell growth, and (**c**) caspase-3/7 activity in PC-3 cells three days after TEA-core G7-mediated delivery of 50 nM Hsp27-targeting siRNA in the presence of 10% serum (hot pink bars in all panels). Data for scrambled siRNA–G7 complexes (gray bars) are shown for comparison. Values are expressed as % relative to control (non-treated cells, green bars). Adapted from [42], with permission of John Wiley and Sons.

#### 3.2.2. In Vivo Data

Encouraged by the results obtained in vitro, high-generation TEA-core dendrimers were tested for in vivo gene silencing performance in various cancer models, including prostate [46], ovarian [47], liver [48], and glioblastoma [49,50]. The application to ovarian cancer treatment represents a particularly interesting example, since, beside the remarkable silencing of AKT—a gene activated in 36% of primary tumors, with the activation being associated with high-grade cancer and aggressive clinical behavior, protection to induced apoptosis, drug resistance and disease relapse [51]—the combined administration of the anticancer drug paclitaxel with AKT siRNA delivered using G6 TEA-core PAMAM nanovectors resulted in a synergistic therapeutic effect in vivo.

In this study, the effective AKT gene-silencing mediated by the G6 dendrimer nanocarriers (Figure 8a,b) was first verified in SKOV-3 cells, a gold standard model for drug-resistant ovarian cancer, as shown in Figure 8c–f. A non-specific (NS) siRNA sequence was also used in all experiments for comparison.

As seen from Figure 8c, the G6 TEA-core PAMAM-mediated gene silencing effect is dependent on siRNA concentration, with approximately 70% inhibition achieved using 50 nM siRNA. Interestingly, AKT knockdown negatively affects the expression of p70S6K, an AKT effector protein involved in cell survival within the phosphatidylinositol 3-kinase (PI3K)/AKT pathway-driven ovarian cancer development [51]. With the same nanovector/siRNA system, the inhibition of cancer cell growth is evident after 72 h post transfection (Figure 8e), and the loss of cell viability can be ascribed to induced apoptosis, as monitored by the significant increase of caspase-3 activity (Figure 8f) [47].

Current chemotherapeutic regimens unfortunately have limited efficacy in patients with advanced ovarian cancer [52]. Paclitaxel is the current first-line drug for treating this malignancy in the clinics, and induced apoptosis has been established as one of the main mechanisms of action of this molecule. Therefore, we speculated that, when administered together, AKT-targeting siRNA and paclitaxel might act synergistically in promoting specific cancer cell death. This hypothesis was initially verified in in vitro experiments, showing how the combined treatment of AKT siRNA/G6 TEA-core dendrimer nanoparticles substantially enhanced SKOV-3 cell growth inhibition via induced apoptosis, as illustrated in Figure 9a,b.

These promising results prompted us to proceed with in vivo experiments. Accordingly, SKOV-3 cells were subcutaneously injected in nude mice, and tumors were allowed to grow until they reached a volume of approximately 30 mm3. The xenografts were then first treated with intratumoral injections of either AKT siRNA/ or non-specific siRNA/G6 TEA-core dendriplexes in the absence of paclitaxel. The differential reduction of tumor volume is well evident in the upper panel of Figure 9c: A tumor shrinkage of ≈50% in mice administered with the targeted siRNA with respect to those receiving the non-specific treatment can be readily appreciated. Concomitantly, immunohistochemistry confirmed the drastic decrease of AKT expression in the AKT knockdown mice (Figure 9d, right panel), the corresponding tumors showing signs of necrosis (Figure 9d, left panel).

**Figure 8.** (**a**) Simulation of G6 TEA-core PAMAM dendrimers in complex with AKT siRNA at N/P = 5. The dendrimers are portrayed as wine-colored spheres, with charged amine groups depicted in pink. siRNA molecules are shown as turquoise sticks. A transparent gray field is used to represent the solvent environment. (**b**) Zoomed view of one single G6 TEA-core PAMAM molecule in complex with one AKT siRNA (colors as in panel a). Some Cl- and Na<sup>+</sup> counterions are shown as white and light gray spheres, respectively; water molecules are not shown for clarity. siRNA concentration-dependent inhibition of AKT (**c)** and its downstream effector p706SK (**d**) in SKOV-3 cells three days after TEA-core G6-mediated delivery (N/P = 5). Data for non-specific (NS) siRNA–G6 complexes are shown for comparison. Protein expression levels were determined by western blotting, quantified by densitometry, and are expressed as fold-change normalized to β-actin. (**e**) Time-dependent growth inhibition of SKOV-3 cells transfected with G6 TEA-core PAMAM dendrimers (N/P = 5) and with non-specific (NS) siRNA–G6 complexes, as determined by the MTT assay. Values are expressed as % relative to control (non-treated cells). Filled bars: 24 h post transfection (p.t.), striped bars: 48 p.t., checked bars: 72 h p.t. (**f)** Caspase-3 activation in SKOV-3 cells determined 72 p.t. with G6 TEA-core PAMAM dendrimers at N/P = 5. Data for non-specific (NS) siRNA–G6 complexes are shown for comparison. Values are expressed as fold change normalized to β-actin used as control. Adapted from [47], which is an open access article published under an ACS AuthorChoice License.

**Figure 9.** SKOV-3 cell viability (**a**) and relative caspase-3 activation (**b**) after transfection with non-specific (NS) siRNA or AKT siRNA delivered by the G6 TEA-core dendrimer nanovectors (N/P = 5) alone or in combination with paclitaxel (100 nM). Non-treated cells and β-actin were used as respective controls. (**c**) Tumor volumes from SKOV-3 xenografted mice treated with non-specific (NS) siRNA (top panel, left) or AKT siRNA delivered by the G6 TEA-core dendrimer nanovectors (N/P = 5) alone (top panel, right) or in combination with paclitaxel (100 nM, bottom panel). (**d**) Drastic reduction of AKT levels in tumor xenografts injected with AKT siRNA G6 TEA-core dendriplexes (**bottom, left**), and the corresponding histological sample showing sign of necrosis (**bottom, right**), compared with xenografts treated with NS siRNA delivered with the same nanovectors showing no reduction of AKT levels (**top, left**) and no necrosis (top, right). (**e**) Tumor volume during combined treatment of AKT siRNA G6 TEA-core dendriplexes/paclitaxel (filled hot pink circles) of SKOV-3 mice xenografts, compared with nanodelivered AKT siRNA (open hot pink circles) or paclitaxel alone (filled gray circles). Data for nanodelivered NS siRNA are shown for control (open gray circles). Adapted from [47], which is an open access article published under an ACS AuthorChoice License.

Finally, the same assays were performed in tandem with paclitaxel administration via mice intraperitoneal injection. The corresponding xenografts reveal a remarkable cooperative action of the two treatments with respect to the chemotherapeutic drug per se (Figure 9c, lower panel), with a reduction of tumor growth of 85% compared to the treatment with paclitaxel alone (Figure 9f) while the G6 TEA-core dendrimer or the AKT siRNA alone have no effect (data not shown). To the best of our knowledge, this constitutes the first study documenting a potentiated anticancer effect of paclitaxel while co-administered with AKT siRNA mediated by dendrimer nanovectors to ovarian cancer both in vitro and in vivo.

#### *3.3. Low-Generation TEA-core PAMAM Dendrimers as E*ff*ective In Vitro and In Vivo siRNA Nanocarriers*

Though the high-Generation G6 and G7 TEA-core dendrimers were very effective in delivering siRNA molecules to various cancer models in vitro and in vivo, the large-scale chemical synthesis of these molecules required for their clinical applications is laborious and very time/resource consuming (e.g., extensive dendrimer purification is intrinsically difficult, being hampered by the presence of highly similar side-products) [53]. These issues imply that the good manufacturing practice (GMP) claimed for products expected to undergo clinical trials is technically very challenging. Therefore, finding a way to endow lower generation dendrimers with effective and efficient siRNA delivery could be a worthy goal per se, as discussed below.

#### 3.3.1. Functional Delivery of Sticky siRNA

In 2007, Jean Paul Behr and his group showed that small interfering RNAs bearing short complementary An/Tn (*n* = 5–8) sticky overhangs delivered using polyethyleneimine (PEI) result in enhanced gene silencing with respect to standard siRNAs [54]. Since PEI is one of the best non-viral DNA carriers but its efficiency drops dramatically during siRNA transfection [55], its ability to quantitatively deliver sticky siRNAs could be attributed only to the presence of the complementary An/Tn overhangs. According to Behr's explanation, the latter can self-assemble into gene-like, long double-stranded RNA, and this in turn allows for the successful cellular delivery by PEI by virtue of a mechanism utterly similar to that governing plasmid DNA transfection. On the other hand, we reasoned that an additional contributing factor to the enhanced gene silencing of nano-delivered ssiRNAs could also be related to the inherent flexibility of the terminal, single-strand nucleic acid fragments; this might allow them to behave as clamps that, just like protruding molecular arms, can better enwrap and tightly hold the nanovector, thereby enhancing binding and, ultimately, delivery. Accordingly, we set on to verify these concepts with the ultimate purpose of exploiting low generation TEA-core PAMAM dendrimers as efficient nanocarriers in RNAi.

#### 3.3.2. In Vitro Preliminary Data of Sticky siRNA Delivery by Lower Generation TEA-Core PAMAMs

Under this perspective, we initially constructed two sticky siRNA molecules with complementary A5/T5 and A7/T7 3 -overhangs to target Hsp27 and TCTP (a highly conserved protein present in all eukaryotic organisms that regulate cell survival in human tumors) in prostate and breast cancer models [56,57]. In parallel, we also synthesized four additional non-complementary ssiRNAs (i.e., A5/A5, A7/A7, T5/T5, and T7/T7) to investigate the effect of chemistry, length and non-complementarity on the relevant gene silencing potential with respect to the standard (A2/T2) siRNAs discussed above [56,57].

Next, we preliminarily tested our TEA-core dendrimers from Generation 4 to Generation 7 for their ability to deliver complementary ssiRNAs in PC-3 cells. Figure 10a shows that, contrarily to conventional A2/T2 siRNA delivery, for which Generation 6 or (better) 7 dendrimers were requested to achieve biological effects (see § 3.2, [42,47]), a significant gene silencing can be achieved in PC-3 cells starting from the G5 nanocarriers, the A7/T7 ssiRNA being somewhat more effective than the A5/T5 counterpart for all nanovector generations (Figure 10a). Since G5 was the lowest and most effective dendrimer generation for the in vitro delivery of ssiRNAs, further investigations were carried out using this nanovector. In particular, the best, long-term gene silencing was obtained with the G5 TEA-core assisted delivery of 50 nM ssiRNas at N/P ratio = 10, as highlighted in Figure 10b.

**Figure 10.** (**a**) Hsp27 gene silencing upon delivery of complementary ssiRNAs mediated by different generations of TEA-core PMAMAM to PC-3 cells. Checked bars: Data for A5/T5 ssiRNA; solid bars: Data for A7/T7 ssiRNA. In these experiments, vinculin was used as reference and non-treated cells were used for control. (**b**) Long-term Hsp27 silencing achieved with A5/T5 and A7/T7 ssiRNAs (50 nM) delivered by G5 TEA-core PAMAMs at N/P = 10. Redrawn from [56], with permission of the American Chemical Society.
