The number of studies that resulted in the successful production of HNSCC organoids is small due to difficult logistics (the time from resection of tumor tissue to delivery to the laboratory must be minimized), an often insufficient volume of biomaterial, as well as high requirements for the qualitative and quantitative composition of the culture medium, which demands the mandatory presence of growth factors, cytokines, and inhibitors of signaling pathways leading to the triggering of apoptosis or epithelial-mesenchymal transition (Table 3). At the same time, the efficiency of obtaining organoids from tumor tissue of the head and neck does not exceed 60–70%, even in laboratories that have been actively working with this material for many years (Table 3).
Thus, obtaining organoids requires significant material and time, as well as highly qualified personnel and close coordination of clinicians and cell biologists. The result is a tumor model that can be maintained in vitro for a long time, while maintaining genetic stability [
118,
119] and fairly fully reproducing the morphological characteristics of the original tissue. For example, organoids from nasopharyngeal carcinoma do not express CK7, unlike organoids from normal mucosa obtained from the same patient, which corresponds to differential expression of this marker in the original tissues [
116]. It has been shown that organoids, when cultivated, retain the expression of many other markers of head and neck tumors, including those used to identify cancer stem cells: CK13, CK18, ALDH1A1, BMI-1, CD44, and CD133 [
116,
117,
119,
120,
121]. Interestingly, lactate, which is promoted by Wnt activity, is required to maintain the population of CD133+ cells in HNSCC organoids. Moreover, silencing monocarboxylate transporter 1 (MCT1), the prominent pathway for lactate uptake in human tumors, with siRNA significantly impaired organoid-forming capacity of oral squamous cell carcinoma cells, which allows MCT1 to be considered as a potential therapeutic target for the treatment of HNSCC [
121].
Despite the ease of production and the possibility of preserving the architecture of tumor tissue, the scope of application of PDE as an in vitro model is significantly limited by its fragility; most often, PDE is used for rapid testing of the effectiveness and/or toxicity of drugs within 5–7 days after being obtained [
129]. It has been shown that the viability of HNSCC explants decreases from 90% to 30% within a week after the start of cultivation [
131]. The survival rate of HNSCC explants can be increased several times by modifying the culture protocol, for example, adding hydrocortisone, aprotinin, ascorbic acid, EGF, or folic acid to the medium [
130,
131], or placing the explant in more physiological conditions that imitate contact with normal tissue, for example, onto the surface of the dermal equivalent [
130] or into cell sheet composing of epithelium and subepithelial stroma [
132]. Under such conditions, explants can be maintained in culture for 21 [
130] or even more than 30 days [
132], while cancer cell heterogeneity and the microenvironment, including vital immune cells, as well as tissue foci of hypoxia, are maintained.
Thus, explants and histocultures of HNSCC are easy to obtain and effective for quickly assessing the response of tumor tissue to drug therapy, which suggests their promise for use in personalized medicine.
When modeling HNSCC, microfluidic devices are most often used for culturing tumor tissue fragments (explants and histocultures) (Table 4).
Thus, tumor-on-chip technology is a valuable tool for personalized assessment of the effectiveness of various antitumor agents (alone or in combination), allowing dynamic monitoring of the response of tissue samples from patients with HNSCC to treatment. The development of microfluidic platforms can improve patient outcomes through the selection of an optimal personalized treatment strategy.
To study the HNSCC biology, several types of in vitro models, differing in many parameters, are currently actively used (Table 5, Figure 1).
The choice of a specific model depends on the tasks and capabilities of the scientific laboratory. As follows from the bibliometric analysis of research papers on various topics in the PubMed database, the number of investigations in the area of 3D cancer models in vitro is growing every year. However, in 2001–2020, only 65 papers for all types of HNSCC 3D in vitro models were published, which is only 0.72% of all studies involving 3D cancer models. Such a small number of works (HNSCC claims the lives of 450 thousand people a year! [
5]) is primarily due to the fact that obtaining a reproducible and relevant 3D model of HNSCC is in itself a rather complex scientific task.
Obviously, 3D models of HNSCC are more valuable from a researcher’s point of view than cell lines or cultures, since they more accurately reproduce the cellular composition and architecture of tumor tissue. These properties allow the use of 3D models to study disease pathogenesis and progression, for which cell–cell interactions and various pathophysiological gradients are important [
142,
143,
144]. Despite varying success rates in generating 3D models, there is consensus regarding their promising potential for testing anticancer agents and enhancing the predictive value of preclinical studies [
43,
67,
119,
120,
145].
As soon as researchers obtained 3D models for their arsenal, there appeared a need for their direct comparison with 2D lines or primary cell cultures in many parameters, i.e., cell morphology, expression of genes involved in carcinogenesis, production of signaling proteins, and response to drug and radiation therapy [
146,
147,
148,
149,
150,
151]. In the vast majority of such works, the authors concluded that 3D models have an undoubted advantage. Below are a few examples. Schmidt et al. examined the ability of 12 HNSCC cell lines to form spheroids in ultra-low attachment plates, showing that the formation of tight regular spheroids was dependent on distinct E-cadherin expression levels in monolayer cultures. After that, microarray analysis was used to create a gene expression array profile of HNSCC cells, growing as a monolayer and as a 3D spheroid. A global upregulation of gene expression was related to genes involved in cell adhesion, cell junctions, and cytochrome P450-mediated metabolism of xenobiotics, whereas downregulation was associated with genes controlling the cell cycle, DNA-replication, and DNA mismatch repair [
152]. In a similar study, Melissaridou et al. compared the properties of five HNSCC lines; when cells transitioned from 2D to 3D culture conditions, there was an increase in the expression of NANOG and SOX2, used as markers of tumor stem cells (for example, for the laryngeal carcinoma line LK1122, expression increased by 26.8 and 22.9 times, respectively), but this did not change the other marker, CD44. The MTS-based assay revealed that cells grown in 3D tumor spheroids showed higher viability after treatment with increasing doses of cisplatin and cetuximab [
146]. The increase in drug resistance when moving from 2D to 3D models has been confirmed by numerous scientific groups [
43,
133]. Typically, we are talking about a 2–20-fold increase in resistance, for example, Cal33 monolayers were 6-, 20-, 10-, and 16-fold more sensitive than spheroids to growth inhibition by ellipticine, idarubicin, daunorubicin, and doxorubicin, respectively [
153]. However, in some studies these numbers are even higher: for a bioprinted 3D model, a 4-fold increase in the IC50 of cisplatin and an 80-fold increase for 5-fluorouracil compared to monolayer HNSCC cultures was shown [
103]. Monitoring treated 3D models allows the observation of the dynamics of drug penetration and distribution gradients, as well as the identification of markers for drug-resistant cell populations that could represent a source of drug failure and recurrence [
147,
153].
4. In Vitro Cell Models of HNSCC and Oncoviruses
The primary risk factors commonly linked to head and neck cancer encompass tobacco, alcohol consumption, using areca nut, and viral infection [
5]. This section considers the development of in vitro models of HNSCC with oncogenic viral infections including human herpes viruses and human papillomaviruses, which play an important role in the pathogenesis of cancer [
5,
154].
4.1. Human Herpes Viruses (HHVs)
Epstein–Barr virus (EBV), also known as human herpes virus type 4 (HHV4), has tropism for B cells and epithelial cells and is closely associated with nasopharynx and oral cavity tumors [
154]. Nasopharyngeal and laryngeal tumor incidence are more associated with the EBV in East Asian populations than with human papillomavirus [
5]. For example, examination of patients with nonkeratinizing nasopharyngeal carcinomas in southern China and Southeast Asia found 100% of them were EBV-positive [
116,
155]. In a study by Wang et al. using the EBV-encoded small RNA in situ hybridization assay, 100% of tumor tissue samples and 100% of organoids obtained from patients with nasopharyngeal carcinoma were EBV-positive [
116
Herpes simplex virus 1 (HSV-1) has been implicated in several diseases of varying severity, such as chronic tonsillitis and HNSCC. The HSV-1-positive cases in HNSCC have been associated with the advanced stage (T3/T4) [
156]. Driehuis et al. monitored the process of infection of patient-derived oral mucosa organoids using tdTomato-labeled HSV-1; it took 2 weeks for the virus to spread throughout the organoids unless inhibited with acyclovir (a viral tyrosine kinase inhibitor) [
123].
4.2. Human Papillomavirus (HPV)
More than 200 papillomavirus types infect humans. According to the World Health Organization International Agency for Research on Cancer (WHO IARC), HPV types 16 and 18 are classified as carcinogenic to humans (Group 1), HPV types 31 and 33 are probably carcinogenic (Group 2A), and HPV types 35, 39, 45, 51, 52, 56, 58, and 59 are possibly carcinogenic (Group 2B) [
154]. Some researchers suggest considering HPV+ and HPV− HNSCC as separate groups of neoplasms. Compared to HPV-negative, HPV+ HNSCC more commonly affects the oropharynx (predominantly), hypopharynx, and larynx than the oral cavity. Moreover, HPV+ HNSCCs have different pathological patterns, immune signature, and mutation burden, i.e., greater infiltration of B cells into the tumor microenvironment, fewer genetic mutations, and intact apoptotic response, which may explain the improved prognosis and superior response to radio- and immunotherapy and significantly longer median survival than HPV-negative HNSCC (130 months vs. 20 months) [
5
Thus, it is not surprising that when modeling HNSCC in vitro, scientists always emphasize the HPV status of the cell line or tumor cells within the 3D model. HPV accounts for 72% of all head and neck squamous cell carcinoma cases in developed nations [
5]; however, in fact, researchers rarely work with HPV+ HNSCC in vitro models. Just compare: the number of currently available HPV+ immortalized HNSCC cell lines is significantly limited, with eight lines known precisely (UMSCC-47, UMSCC-104, UPCI:SCC090, UPCI:SCC152, UPCI:SCC154, 93-VU-147T, HMS001, and LU-HNSCC-26) [
20,
120,
157
The development of primary cell lines from a naturally infected HPV+ cancer is rarely successful [
21] due to the low mutational load; for example, the TP53 gene is almost always wild-type, both in tumor tissue and in primary cell cultures or immortalized cell lines derived from it [
120,
157]. The preservation of genes involved in cell cycle control and apoptosis induction leads to low proliferation rates of HPV+ tumor cells when transferred to a 2D monolayer; the cultures are highly radiation and drug-sensitive and readily enter senescence. It is important to note that immortalized HPV+ cell lines retain the radiation sensitivity both in 2D form and when cultured in the form of spheroids or cell line-derived xenografts [
158,
159]. Interestingly, some HPV+ immortalized lines are unable to form stable 3D spheroids (for example, UMSCC-47) or only form slowly growing spheroids (for example, UPCI:SCC090) [
159
Similar difficulties are observed with patient-derived 3D models. For a long period of time, the attempts to obtain organoids from HPV-associated HNSCC were unsuccessful. This was primarily for clinical reasons, i.e., the patients with HPV+ HNSCC less commonly receive surgery. One of the first organoids from HNSCC specimens was obtained by Tanaka et al., with the efficiencies similar for HPV+ (3/9) and for HPV− (10/34) patients [
120]. In another study, the efficiency of obtaining organoids was higher: eight lines of HPV+ organoids were isolated from nine samples of initially infected tumor tissue [
160]. Another approach for obtaining organoids from HPV+ tumor tissue involves the implantation of tumor fragments in NOD/SCID/IL-2Rγ−/− mice to a <25% stable engraftment rate; after the xenograft reaches a volume of 1 cm
3, it was passaged at least twice, removed, dissociated into individual cells, and cultured on ECM to form organoids. The approach originally yielded a panel of nine HPV+ organoids from nine xenografts [
161
HPV+ organoids can also be obtained by infection of oral mucosa organoids. Driehuis et al. used HPV16 particles, which led to accumulation of viral DNA in organoids, and after 12 days the presence of virions was confirmed in a conditioned medium [
119]. Such models are undoubtedly easier to obtain and suitable for studying the pathogenesis of HPV-associated HNSCC.
Another available HPV+ in vitro model involves patient-derived explants; culturing on a supporting matrix (dermal equivalent) allows maintaining the viability of a tumor tissue fragment for up to 14–21 days depending on the sample. In this model, infection with the virus, confirmed by detection of HPV DNA or IHC staining for the p16INK4a marker, persists even after irradiation [
130].
5. New Trends in In Vitro Modeling of HNSCC
The development and implementation of a personalized approach in clinical oncology requires constant improvement of preclinical models aimed at maximizing the approximation of cell models to the structure of the native tumor and increasing the efficiency of obtaining such models. For in vitro models of HNSCC, several areas of research can be distinguished.
5.1. Models of Vascularization
In the tumor in vivo microenvironment, there is an active interaction between tumor and endothelial cells, which is important for the recruitment of angiogenic cells, tumor cell survival, and migration [
123,
162]. In most cases, the method of co-cultivation of 2D cultures is used to analyze the interaction of tumor and endothelial cells [
163]. It has been shown that exosomes produced by PCI-13 (HPV−) and UMSCC47 (HPV+) cell lines, as well as exosomes from plasma of HNSCC patients, stimulated proliferation, migration, and tube formation by human umbilical vein endothelial cells (HUVECs) in vitro and promoted formation of defined vascular structures in vivo [
164]. To improve inter- and intratumoral vasculature, tumor cells actively produce proangiogenic factors. When 2D HNSCC cell lines are transferred to 3D culture conditions, their proangiogenic potential increases; transplantation of a suspension of primary cancer-associated fibroblasts and 3D spheroids from FaDu cells led to the growth of a well-vascularized tumor in a mouse model of xenografts, while, when transplanting cell suspensions from 2D cultures, gradual necrosis of the graft due to insufficient blood supply to the tissue was observed [
165]. To our surprise, we found a description of only one vascularized 3D model of HNSCC. Bessho et al. created a tissue-engineered model of oral cancer; human oral squamous cell carcinoma (HSC-4) cells, human umbilical vein endothelial cells (HUVECs), and normal human dermal fibroblasts were successfully cocultured within gelatin-based matrix, resulting in structures that mimicked 3D-cancer tissues. This model was used to assess sensitivity to X-ray irradiation [
166
Angiogenesis in solid tumor tissue is a promising target for HNSCC-targeted therapy. Researchers are actively searching for signaling molecules and pathways of interaction between epithelial and tumor cells, and tumor-associated lymphocytes (including B cells and T cells) [
167] or macrophages [
43] can act as mediators in this interaction. To create a relevant model that is as close as possible to native tissue, it is necessary to reproduce in vitro one of the key processes of tumor progression and metastasis-angiogenesis [
168,
169]. The furthest along this path has been bioprinting technology, which can directly bioprint the vascular wall structures or create hollow channels inside a volumetric matrix that will later be populated by endothelial cells [
162]. Microfluidic technologies also make it possible to study various aspects of the interaction between tumor and endothelial cells in vitro: metastasis (intra- and extravasation), chemotaxis, production of angiogenic factors, etc. [
170,
171]. New prospects, in our opinion, are also opening up for patient-derived explant cultures: culturing an explant of tumor tissue on an artificial vascular bed (co-culturing vascular endothelial cells and smooth muscle cells in a matrix comprising ECM components and growth factors) can increase the duration of its existence ex vivo [
129].
5.2. New Types of Matrices for Tumor Cell Culturing
Currently, there is an active process of abandoning xenogeneic reagents when modeling human tissues in vitro in favor of autogenous or allogeneic materials; it is assumed that such a replacement will make it possible to recreate a more favorable microenvironment for cells, simulating their original environment in vivo [
172]. Thus, when modeling HNSCC in vitro, it was proposed to use allogeneic gels based on human tumor tissues—human uterine leiomyoma-derived Myogel or human pre-metastatic neck lymph node-derived Lymphogel, instead of Matrigel (solubilized basement membrane preparation extracted from the Engelbreth-Holm-Swarm mouse sarcoma) [
173,
174]. Such matrices differ in their protein composition, including the content of unique adhesion-related proteins [
174]. An experiment involving 12 HNSCC cell lines revealed an increase in the resistance of tumor cells to the action of EGFR and MEK inhibitors compared to cells cultured on plastic or in Matrigel. The authors of the work suggest that the human tumor matrix improves the predictability of in vitro anticancer drug testing [
173].
6. Three-Dimensional In Vitro Cell Models of HNSCC for Personalized Medicine
Currently, the attention of researchers is aimed at the development of personalized medicine, where an important tool is 3D models obtained from tumor tissue of patients. The main goal is to obtain a model that can predict the patient’s response to therapy in order to optimize the treatment strategy [
175
Valuable information for clinicians can be obtained by using patient-derived explants (including those cultured using microfluidic devices) to test antitumor therapy [
176]. Such studies already make it possible to identify inter- and intra-patient variability in response to irradiation and chemotherapy [
138
Tanaka et al. [
120] managed to obtain HNSCC organoids that were as close as possible to the primary tumor; they had a similar morphology, and retained the expression of markers of mesenchymal (vimentin) and tumor (CD44 and ALDH1A1) cells, although CD68+ cells were found only in the tumor, but not in the organoids. When testing sensitivity to platinum and docetaxel therapy in the resulting organoids, consistency was found between the response in vitro and in vivo [
120
Driehuis et al., analyzing the expression of markers TP63, TP40, Ki67, CK13, and CK5, convincingly showed that organoids obtained from the mucous membrane of the oral cavity or tongue recapitulate the functional and morphological characteristics of the tumor [
119,
177]. The same study found that the response of organoids to radiation correlated with the response of patients; three organoid lines were among the most resistant when exposed to radiotherapy in vitro, and three corresponding patients relapsed after undergoing radiotherapy; moreover, a patient whose organoid line showed the highest sensitivity to irradiation had a lasting response to palliative radiotherapy [
119
Millen et al. describes the organoid biobank, which stores a collection of biomaterial from patients with HNSCC from various anatomical locations and histological cancer subtypes. During routine surgical resection or biopsy procedures in 2019–2022, samples were collected from 228 patients, but organoids were successfully obtained from only 97 patients (42.5% efficiency). To store organoids, the biobank uses cryopreservation technology; the proportion of successfully thawed organoids was 70.9%. The resulting organoids retained histopathological and molecular features of primary tumor tissue; tumor cells in their composition expressed CK13 and p63, and a genetic study revealed mutations in the tumor-associated genes TP53 (in 63% of organoids), NOTCH1, PIK3CA, FAT1, and APOB, gains of oncogenes including PIK3CA, FGF3, and FGF4, and loss of tumor suppressor CDKN2A. The collection included unique samples, for example, four salivary gland tumor organoid models that retained the ability to produce mucin and amylase, as well as organoids from tissues of a patient with Fanconi anemia, which had increased sensitivity to double-stranded DNA breaks induced by mitomycin-C. Despite the similarity of organoids and tumor tissue at the cellular and molecular levels, in combined treatment (radiation therapy + cisplatin) no clear correlation between organoid and patient response was observed. At the same time, the organoid response to radiotherapy (measured as organoid viability at 2 Gy) correlated with clinical relapse status in the adjuvant setting but not in the primary setting. In addition, the researchers successfully used CRISPR-Cas9 base-editing technology to introduce the E545K mutation (one of the most common PIK3CA mutations), which opens up new opportunities for scientists to use patient-derived 3D models to search for and validate potential biomarkers of HNSCC response to therapy [
160].
Promising results of experimental work allowed scientists to move on to promoting clinical trials using in vitro models of HNSCC. In October 2023, only three clinical trials were registered, which is entirely attributable to the difficulty of obtaining patient-derived models of head and neck cancer.
The ORGAVADS (ORGAnoids + VADS (French “voies aérodigestives supérieures”) tumors) study is a multicenter observational trial (NCT04261192) conducted to investigate the feasibility of generating and testing patient-derived tumor organoids derived from HNSCC for the evaluation of sensitivity to treatments (chemotherapy, radiotherapy, PARP inhibitors, and immunotherapy). The planned number of study participants is determined taking into account that the expected efficiency of obtaining organoids from patient material will be about 60%. The sponsor (Centre Francois Baclesse) assumes that this screening could make it possible to refine the choice of treatments adapted to each patient and thus limit the undesirable effects [
178,
179].
Investigators hypothesize that high-throughput screening on patient-derived tumor organoids can be used as an adjunct tool to aid treatment selection in patients with cancer. The objective of the NCT04279509 study is to determine if a drug screen assay (panel of ten primary and five additional anticancer drugs) using personal organoid models can accurately select a chemotherapeutic agent that results in objective response in patients with refractory solid tumors (including HNSCC) [
180].
The SOTO (Sensitivity of Organoids to Treatment Outcome) prospective observational study (NCT05400239) aims to determine the sensitivity of organoids obtained from patients with HNSCC to radiotherapy, platinum (cisplatin and/or carboplatin) chemotherapy, or cetuximab or their combination, with subsequent determination of the correlation of the treatment sensitivities of organoids with the treatment outcome of patients [
181].
If successful, the clinical trials described above will mark a new stage in the development of personalized medicine in head and neck cancer.
7. Conclusions
For several decades, researchers have been trying to answer the demand of clinical oncologists to create an ideal preclinical model of HNSCC that is accessible, reproducible, and relevant. Over the past years, the development of cellular technologies has naturally allowed us to move from short-lived primary 2D cell cultures to complex patient-derived 3D models that reproduce the cellular composition, architecture, mutation, or viral load of native tumor tissue. The variety of models developed in vitro allows scientific teams to solve a wide variety of problems: screening agents with potential antitumor activity, studying the contribution of the tumor microenvironment to its progression and metastasis, determining the prognostic significance of individual biomarkers (including using genetic engineering methods), studying the influence of viral infection on the pathogenesis of the disease, and adjusting the treatment tactics for a specific patient or groups of patients. Promising experimental results have created a scientific basis for the registration of several clinical studies using in vitro models of HNSCC. It can be assumed that in the coming years, the field of in vitro modeling of tumor tissues will actively develop: biobanks of such models will become widespread, new biomarkers of HNSCC with high predictive value will be identified, and the proportion of successful clinical trials and the effectiveness of treatment will increase due to more accurate results of preclinical studies.