Current therapy of pancreatic cancer

Pancreatic cancer (PC) accounts for approximately 7% of all cancer deaths worldwide and 3% for cancer deaths within the United States [5], making it the $5\text{th}$ most common cancer-related death in the United States and the $4\text{th}$ most common cause of cancer-related deaths in Europe [6]. The average 5-year relative survival rate for all localization stages of pancreatic cancer is 5–9% [7]. When detected in its early stages, the surgical removal of the cancerous tissue is often the most promising treatment available. Neoadjuvant (given before surgery with the intent to cure) chemotherapy is administered to improve the likelihood of complete tumor removal and mitigate micrometastatic growth. Adjuvant chemotherapy (administered after surgery) is given in hopes of controlling micrometastatic disease. Radiation therapy may be applied to control the disease locally before or after surgery. Palliative chemotherapy is provided to patients with inoperable disease in an attempt to control symptoms and delay death. Current chemotherapy treatments often entail combined regimens such as FOLFIRINOX (5-fluorouracil [5-FU], leucovorin, irinotecan, oxaliplatin) which demonstrated improved post-surgical survival [8]. Other chemotherapeutic drugs include $capecitabin{e}^{\mathit{TM}}$, $paclitaxe{l}^{\mathit{TM}}$ (PAX), $erlotini{b}^{\mathit{TM}}$ and $gemcitabin{e}^{\mathit{TM}}$(GEM). In 2019, the polymerase inhibitor $olapari{b}^{\mathit{TM}}$ was approved by the U.S. Food and Drug Administration (FDA) for metastatic PC [9].

Though some of these newer agents and regimens have improved survival, this has been on the order of only a few months and progression of the disease occurs in more than 90% of patients. Thus, it is necessary to explore alternative treatments such as NP-based therapies that are able to more effectively target and treat complex cancerous tissues. Targeted therapies are in development which could address the unique challenges presented by pancreatic adenocarcinoma (PA). Novel agents are being designed which modify various processes thought to be responsible for PA virulence. The targeted mechanisms include signaling pathways such as the ones mediated through phosphoinositide 3-kinases (PIK3), supportive processes such as angiogenesis and the desmoplastic environment, immune response augmentation, and agents which may work towards interrupting the epithelial to mesenchymal transition.

To date, only a handful of NP-based therapies have been approved for clinical use. One such example is the nanoliposomal irinotecan, a topoisomerase inhibitor, which acts as a second line of treatment following GEM-based chemotherapy for advanced PC [10–13]. Furthermore, NP-bound albumin with encapsulated PAX may be used in combination with GEM and has been approved as a standard therapy with evidence of increased tumor stroma depletion [14]. These NP-based treatments demonstrate promise as further research is performed within the field of NP-based cancer therapies.

Current treatment of bile duct carcinomas

Cholangiocarcinoma is a malignancy originating from the biliary epithelium and develops anywhere along the biliary tree. The overall prognosis for cholangiocarcinoma remains poor with a 5-year survival rate ranging between 8 and 24% [7], depending on the stage of the disease at the time of discovery. Similar to PC, the most effective treatment for advanced bile duct carcinomas (BDC) is surgery, particularly surgical resection. Despite chemoresistance present in BDC, the ABC-02 trial confirmed that GEM in combination with cisplatin could be used for advanced unresectable BDC and was effectively able to prolong patient survival by an average of 4 months [15]. Further treatment options for biliary tract cancers include adjuvant combination therapy performed for at least 6 months with capicitabine, FU-5, paclitaxel or irinotecan. Moreover, the use of nab-PAX and GEM have emerged as the standard chemo-agents in co-formulating therapeutics for front-line treatments. Neoadjuvant application of the substances was shown to increase treatment success and survival after surgery in BDC [16]. In 2020, the FDA approved $pemigatini{b}^{\mathit{TM}}$, which targets the hyperactivity of the oncogenetic fibroblast growth factor receptor 2 (FGFR2)—for unresectable advanced BDC [17]. Current standard of care plans may include chemotherapy; however, surgery remains the primary treatment for a non-metastatic disease. Depending on the location of the tumor, intraductal ablation therapy can be administered by placing a radioactive probe at or near the tumor site. Meanwhile, unresectable and/or metastasized BDC are treated with external radiation therapy. A uniform treatment strategy remains to be developed for adjuvant and neoadjuvant therapies. Due to intrinsic drug resistances, research on novel agents and drug combinations is fundamental for the cure of BDC.

Metallic & inorganic particles

Metal nanoparticles have gained considerable attention due to their promising properties and therapeutic applications in cancer treatment. Materials such as gold or iron oxides have revolutionized the approach to cancer therapy. Gold nanoparticles (GNPs) are established nanostructures that strongly absorb light, allowing them to generate thermal energy often resulting in the photothermal destruction of cancerous tissue [24]. Photothermal ablation occurs when GNPs are excited with a wavelength that corresponds to the specific surface plasmon resonance of the particle [25]. This is particularly desirable in GI carcinomas since surgical removal of these tumors is either too complicated or not possible for the majority of patients. Furthermore, GNPs are biocompatible and are considered non-toxic, making them desirable candidates for the targeted delivery of therapeutic chemicals [26]. At present, combination strategies involving both photothermal therapy and chemotherapy are used to increase the response of PC to chemoactive drugs [27]. This synergistic approach shows potent anti-tumor activity. Photothermal ablation is achieved via GNPs, selenium NPs, or copper sulfide NPs [28–30]. Another recent study demonstrated the application of GNPs when adhered to the surface of PAX loaded poly(lactic-co-glycolic acid)(PLGA)-microspheres [31]. Further research describes conjugated poly(ethylene glycol)-polylactide (PEG-PLA)-GNPs with doxorubicin (DOX), a tyrosine kinase inhibitor, and varlitinib, an anthracycline [32]. Upon radiation with near infrared light, the drug system presented an elevated cytotoxic effect on a PC cell line when compared to the effects seen without plasmonic activation of GNPs.

Additionally, iron oxides such as magnetite ($F{e}_3{O}_4$) exhibit magnetic properties that can be exploited for guidance in enhanced tumor thermotherapy [33]. One major concern when using this material is the release of toxic hydroxyl radicals. Therefore, chemically functionalized maghemite ($\gamma \text{-}F{e}_2{O}_3$) is used as an alternative in drug delivery systems targeting tumor cells via material surface modifications [34]. Recently, iron oxides have been conjugated with liposomes to create magnetoliposomes that are able to deliver chemical agents via selective drug release when exposed in a magnetic field [35]. Endeavors to create versatile and reliable delivery systems have led to inorganic carriers composed of carbon or silica-conjugates such as siliciumdioxide. For example, mesoporous silica NPs have been proposed as efficient delivery structures for therapeutic agents with peculiarities comprising of increased drug solubility, high loading capability, multifunctionality as well as stimuli-responsive release control. Among various beneficial characteristics, silica NPs are especially compelling for GI cancer applications since their surface can be easily modified to penetrate through GI barriers or target cancerous cells [36, 37]. These studies show tremendous promise in ongoing efforts to create reliable targeted delivery systems.

Yet another inorganic NP delivery system is the metal organic framework (MOF), which is constructed to perform a variety of functions [38]. MOFs are hybrid nanomotifs built from metal ions embedded in a network of organic linkers and recent evidence indicates that they are effective with respect to photodynamic therapy, as well as enhanced immunotherapy [39, 40]. Moreover, MOFs have a high drug-loading ability, biocompatibility, and multifunctionality. Evidence of this is provided through the application of stimuli-responsive MOFs with various functionalized coatings for programmable nanodelivery [41]. The study explored different MOFs, including zeolitic imidazolate frameworks-8 (ZIF-8), the UiO-66 (Universitetet i Oslo) framework, and MIL-101 (Matérial Institut Lavoisier), a combination of terephtalic acid and chromium salts. The NPs were coated with polydopamine (PDA), an oxidation product of catecholamines, which has excellent photothermal transduction properties. Conjugation of chemotherapeutics with PDA results in better solubility, as well as a more controlled drug release providing the opportunity to target tumor cells via chemo-photothermal destruction [41]. Additional research has been conducted linking UiO-66 MOFs to cytosine-phosphate-guanosine (CpG) as cancer vaccine adjuvants [42] while folic acid and HER2-based therapies have also been explored as a proof of concept with nanoparticles [43, 44].

Inorganic NPs, such as the ones described above may offer advantages over polymeric particles with respect to size and shape control, as well as the simplified preparation and functionalization of NPs. Additionally, these particles may be tracked more easily via magnetic resonance imaging (MRI), analytical techniques such as mass spectrometry, or optical techniques performed at the cellular level [45]. Yet, inorganic NPs may also have disadvantages related to degradability, elimination through the kidneys and oxygen radical formation leading to potential toxicity.

Viral nanoparticles

Viruses may also be used to develop technologies in the fields of biomedicine and nanoparticles, particularly in tissue targeting and drug delivery. Viral nanoparticles (VNPs) are robust in their protection of nucleic acids due to the stability of the capsid. This increases the resistance of VNPs to both temperature and pH levels while allowing the VNPs to remain stable in a variety of solvents. Additionally, VNPs are often considered symmetrical, polyvalent, and monodisperse. Numerous VNP platforms have been developed, including icosaherdral plant viruses and bacteriophages, as well as rod-shaped plant viruses and filamentous phages [46]. The adaptability of these platforms allows modifications to be made such that binding of drug molecules can occur via encapsulation, adsorption, or covalent attachment.

Cargo molecules may be encapsulated through interactions with the interior architecture of the capsid, whose highly symmetric and repetitive surface provides multiple sites for the covalent attachment of site-specific residues [47]. Therapeutic drugs and imaging agents can then be chemically attached to reactive functional groups such as thiol for specific delivery [48–50]. The efficacy of this method has been validated through the use of a virus-like protein cage architecture used to attach and release the anticancer drug DOX [51]. This approach of drug delivery is advantageous because the protein cage protects the therapeutic compound from the exterior environment, making it inert until the correct time and location for drug delivery is reached.

Viruses may also present molecules exposed on their surface that allow for host recognition, implying that the virus may avoid host defense mechanisms. Additionally, viruses can also have an affinity for receptors of biomolecules that are up-regulated in tumor cells [52]. To combine these two desirable qualities, ligands may be attached to the exterior of the viral surface, incorporating a variety of antibodies, targeting molecules, and peptides onto the surface of the capsid using chemical or genetic means of adhesion [53, 54]. Such an approach has been shown to provide cell-specific and tissue-specific targeting with applications in the delivery of therapeutics or imaging agents [55, 56]. In vivo tumor targeting has utilized antibodies such as transferrin, folic acid, and single-chain antibodies [57]. Similarly, thiol groups may be exposed on the surface allowing for viral capsid monolayers to be patterned on gold surfaces [58]. Furthermore, amino acid residues are used to modify the protein cage architecture so that the reactive sites are available to bind small molecules for site-specific attachments on gold nanoparticles [59], fluorophores [48, 50], carbohydrates [60], nucleic acids [61], and peptides [55, 56]. Additional VNP platforms and their applications in drug delivery and imaging are reviewed in prior work [47, 57, 62–68]. Further research on synthetic compounds may provide a broader range of possible small molecules that could be attached for delivery via virus-based NPs [69, 70].

Polymers

Polymers are large molecules consisting of recurring structural units (known as monomers) that are joined with covalent bonds and can also be utilized as a vehicle for drug delivery to treat cancer. Polymers have several beneficial qualities with regard to drug delivery, patient safety and manufacturing.

During preparation of polymer-based systems for drug delivery, the drug is either entrapped or covalently bound to the matrix of the polymer [71]. Thus, there are multiple resultant structures such as polymeric NPs, micelles, or dendrimers [53]. Schematic representations of some of these systems are shown in Fig. ​Fig.1.1. Polymers used in drug delivery systems can be divided into two categories: natural and synthetic. Example compounds for each polymeric system of drug delivery in GI cancers are provided in Table ​Table22.

Polymeric micelles

The second polymer-based drug delivery system are polymeric micelles, whose functional properties are based on amphiphilic block copolymers [53]. When assembled, these copolymers form a nano-sized shell within an aqueous solution. The hydrophobic core serves as a reserve for the hydrophobic drug being delivered. Drugs can be loaded into the polymeric micelle in two ways: encapsulation [89] or covalent attachment [90]. Thus, the polymeric structure is water-soluble and can be delivered to a patient through intravenous administration [91].

Only few micelles for cancer therapy have reached clinical practice. Presently, polymeric micelle delivery systems are faced with drawbacks such as non-targeted delivery, difficulties in creating multidrug systems and reduced drug resistances. However, micellar drug formulations have potential for future clinical use with emergence of one of the most successful nanocarriers: PEG–PLA micelles [92]. Functionalized PEG-PLA micelles are capable of delivering chemotherapeutics, photothermal and photodynamic therapy components, tumor-associated antigens, nucleic acids, and anti-tumor agents such curcumin or doxorubicin during antivascular therapy [93]. Current clinical trials are testing multiple untargeted PEG-PLA micelles containing various drug compositions [94]. Further developments should aim to overcome drug resistances and provide active tumor targeting, and combine multiple active agents. In efforts to pursue these goals, arginine-glycine-aspartic acid (RGD)-decorated PEG-PAX conjugates have been designed for the treatment of gastric cancer [95]. These conjugates assemble autonomously into micelles that are able to release PAX in weakly acidic environments. Further toxicity studies must be performed to integrate micelles into approved treatment regimens.

NP-based therapies for pancreatic and bile duct cancer

Due to a limited number of treatment options with low treatment efficacy, PC remains one of the most fatal cancer, resulting in a patient 5-year survival rate of less than 5%. Therefore, significant emphasis has be placed on developing more targeted and efficient therapies. Nanosystems have demonstrated enhanced bioavailability by promoting prolonged drug circulation without degradation [139]. Moreover, certain NPs are able to increase the penetration rate of the drug into cancerous cells. Notably, collagenase NPs combined with PAX-micelles have been used in pancreatic ductal adenocarcinoma treatments to improve anti-tumor activity [136]. The proteolytic-enzyme complex encapsulated in a $100\text{nm}$ liposome cleaves excessive extracellular matrix and facilitates drug invasion. Meanwhile, PEG-PLGA NPs coated with neutrophil membranes were shown to overcome the blood-pancreas barrier and actively accumulate in cancerous tissue of a mouse model [140]. The bio-mimicking nanocarriers were loaded with celastrol, a pentacyclic triterpenoid extracted from Tripterygium wilfordii. Celastrol displays anti-inflammatory properties by inhibiting the activation of nuclear factor $\kappa$ light chain enhancer of activated B-cells (NF-$\kappa$B) and acts as an anti-cancerogenic agent, which locally reduced tumor size and inflammation.

Further enhancements in chemotherapeutics have been exhibited through the application of cationic albumin NPs loaded with hydrophobic celastrol and hydrophilic 1-methyltryptophan as a combination therapy with an indoleamine-2,3-dioxygenase (IDO) inhibitor [141]. Recent work describes the use of superparamagnetic iron oxide NPs (SPION) linked to Curcumin, a bioactive anti-cancer agent with promising effects in fighting malignant tumors [142]. Furthermore, chemoresistance to gemcitabine and tumor sphere formation were significantly reduced following the administration of Curcumin via SPION in an orthotopic mouse model [143]. GEM efficiency was also enhanced using the novel PAX nanoformulation in PLGA-NPs [144]. In this particular study, the lipid synthesis that leads to tumor survival and progression was inhibited by PAX delivered specifically to pancreatic cells. Similarly, GEM was delivered to pancreatic tumors with activatable liposomes [145]. These medical procedures are currently in clinical trials where the prevailing conclusions is that NP-based drug delivery methods are more effective in treating GC [146].

NP-based therapies for gastric & esophageal cancer (GC)

As with other upper GI carcinomas, gastric cancer develops asymptomatically and is a significant cause of cancer-associated deaths. However, NPs can be effectively employed in the early detection of GI cancer. GNPs-RNA conjugates are used in the coloriometric detection of biomarker micro-RNA in GC [147]. In the treatment of advanced GC and its metastasis, evidence indicates the improved effectiveness of nab-PAX [148, 149]. These formulations could be used to increase the therapeutic value of the drugs by increasing the solubility. Interestingly, the binding of NPs to mucous tissues during oral administration sufficiently attenuated the progression of Helicobacter pylori-associated GC, which constitutes the root cause of approximately 60% of all GC [150]. Additionally, gene therapy consisting of exosomic NP-delivery of anti-mIR was used to reverse chemoresistance to cisplatin in GC [151]. As overexpression of HER-2 represents the major cause of GC development, the corresponding gene is an important target in GC therapy. While exploiting the photodynamic properties of gold, Zhang et al have suggested gold nanoshells for simultaneous delivery of HER-2-siRNA and photothermal cancer ablation [152]. Others demonstrated the delivery of gastric stem cell markers via specific antibodies loaded in PLGA-PEG-NPs in an attempt to create reliable target modalities [153]. As for other cancer types, there is ongoing research on the effect of curcumin against cancer cells in various animal models. Promising studies for GC include phospholipid-coated mesoporous silica NPs that have been simultaneously loaded with PAX and Curcumin to treat orthotopic mice models [154]. Alginate and cellulose have gained noteworthy attention for being mucoadhesive and therefore suitable canditates for gastric malignacies. There are various preclinical studies examining the effectiveness of PAX-ALG-NPs, Curcumin-alginate-NPs, ($F{e}_3{O}_4$)-carboxymethyl cellulose NPs loaded with 5-FU on in vitro and in vivo GC models [155–157]. Fighting drug resistance, increasing the solubility and availability of the agents and reaching the GC tumorous cells in an effective manner remain major concerns in improving GC therapeutics.

Not applicable.