Cytokine-enhanced vaccine and suicide gene therapy as surgery adjuvant treatments for spontaneous canine melanoma

Intestinal Probiotic Lysate Modified Bifunctional Nanoparticle for Efficient Colon Cancer Immunotherapy

Background: In cancer immunotherapy, gene therapy has become a promising strategy through the introduction of immunostimulatory components into its formulation. However, ideal non-viral gene delivery platforms capable of simultaneously maintaining a high delivery efficiency and immune activation are still in demand. As an intestinal probiotic, Lactobacillus reuteri has potential correlation with cancer progression. Its unique antigenicity also confers its immunomodulatory activity. Method: We engineered a new non-viral siRNA delivery system, DMPLAC. By wrapping the lysate of Lactobacillus reuteri, it is expected to enhance the anti-cancer immunostimulatory properties. Result: Supported by certain internalization pathways, the prepared DMPLAC nanoparticles showed high siRNA delivery efficiency in vitro (up to 97.62%). They also strongly promoted the maturation and activation of immune cells, including dendritic and T cells, both in vitro and in vivo. By loading siRNA targeting the immune checkpoint CD47 gene, the DMPLAC/siCD47 complex strongly suppressed the growth of multiple colon cancer models through local administration with high safety. Conclusions: Our study developed a novel intestinal probiotic lysate-based gene delivery system with dual immunomodulatory abilities, suggesting a potential strategy for cancer immunotherapy.

Intestinal probiotic-based nanoparticles for cytotoxic siRNA delivery in immunotherapy against cancer

Immunogene therapy has emerged as strategy against cancer by introducing immune-stimulating components into gene therapy. However, there is still a need for an ideal platform to achieve both immune stimulation and efficient gene delivery. Lactobacillus reuteri has potential immunomodulatory activity owing to its unique antigenicity, which is potentially relevant to cancer progression. Here, we designed a novel non-viral siRNA vector (DMPLAC) by encapsulating Lactobacillus reuteri lysate in DMP. DMPLAC can promote maturation and activation of immune cells, increase infiltration of APC and cytotoxic T cells in tumor microenvironment, and exhibit tumor suppressive effects. Loading of siRNA targeting Stat3, DMPLAC/siStat3 further inhibits tumor in multiple models. We designed a strategy that combines immune activation with Stat3 silencing, triggering an immune response and tumor killing. This dual-functional design provides a new choice in development of effective immunogene therapy.

Cancer Immunotherapy

The enhanced understanding of immunology experienced over the last 5 decades afforded through the tools of molecular biology has recently translated into cancer immunotherapy becoming one of the most exciting and rapidly expanding fields. Human cancer immunotherapy is now recognized as one of the pillars of treatment alongside surgery, radiation, and chemotherapy. The field of veterinary cancer immunotherapy has also rapidly advanced in the last decade with a handful of commercially available products and a plethora of investigational cancer immunotherapies, which will hopefully expand our veterinary oncology treatment toolkit over time.

Proliferation index and pseudoprogression as predictors of the therapeutic efficacy of suicide gene therapy for canine melanoma

In our veterinary clinical trials, the combination of systemic immunotherapy with local herpes simplex virus thymidine kinase/ganciclovir suicide gene (SG) treatment induced tumor pseudoprogression as part of a strong local antitumor response. This phenomenon could be owing to tumor inflammation, increased vascular permeability and to different tumor growth rates before, during and after SG therapy. The proliferation index (PI: the fraction of viable cells in S, G2/M, and hyperdiploid phases) would reflect the in-vivo and in-vitro proportion of proliferating melanoma cells in the absence of treatment (PIB) or in response to SG (PISG). The extent of in-vivo and in-vitro melanoma cells responses to SG exhibited a reverse correlation with PIB and a direct correlation with PISG. Then, the final SG outcome depended on the balance between PIB-dependent 'regrowth resistance' versus 'regrowth sensitivity' to SG treatment. In all the cell lines derived from canine tumors presenting partial responses to SG treatment, PISG prevailed over PIB. Conversely, as more aggressive was the tumor (greater PIB of the cell line), the more the balance displacement towards 'regrowth resistance' over SG 'regrowth sensitivity'. All these parameters could have a prognostic value for SG treatment response and provide a glimpse at the clinical benefit of this therapy.

Cancer Immunotherapies

The enhanced understanding of immunology experienced over the last 4 decades afforded through the tools of molecular biology has recently translated into cancer immunotherapy becoming one of the most exciting and rapidly expanding fields. Human cancer immunotherapy is now recognized as one of the pillars of treatment alongside surgery, radiation, and chemotherapy. The field of veterinary cancer immunotherapy has also rapidly advanced in the last decade with a handful of commercially available products and a plethora of investigational cancer immunotherapies that will hopefully expand the veterinary oncology treatment toolkit over time.

Combination of cytokine-enhanced vaccine and chemo-gene therapy as surgery adjuvant treatments for spontaneous canine melanoma

After 6 years of follow-up treating 364 canine melanoma patients, we present here results about the proof-of-concept, safety, and efficacy of a new surgery adjuvant combined gene therapy. The adjuvant treatment (AT) group was divided in three arms as follows: (i) complete surgery plus vaccine (CS-V), (ii) complete surgery plus combined treatment (CS-CT), and (iii) partial surgery plus combined treatment (PS-CT). Besides the genetic vaccines composed by tumor extracts and lipoplexes carrying human interleukin-2 and granulocyte-macrophage colony-stimulating factor genes, the patients were subjected to combined treatment received in the post-surgical bed injections of lipid-complexed thymidine kinase suicide gene plus ganciclovir and canine interferon-β gene plus bleomycin. As compared with surgery-only treated controls (So), CS-CT and CS-V treatments significantly increased the fraction of local disease-free (from 20 to 89 and 74%) and distant metastases-free patients (M0: from 45 to 87 and 84%). Although less effective than CS arms, PS-CT arm demonstrated a significantly improved control of metastatic disease (M0: 80%) compared with So (M0: 44%). In addition, AT produced a significant 9.3- (CS-CT), 6.5- (CS-V), and 5.4-fold (PS-CT) increase of overall survival as compared with their respective So controls. In general terms, the AT changed a lethal disease into a chronic disease where 70% of CS-CT, 51% of CS-V, and 14% of PS-CT patients died of melanoma unrelated causes. These surgery adjuvant treatments delayed or prevented post-surgical recurrence and distant metastasis, and improved disease-free and overall survival while maintaining quality of life. These successful outcomes encourage assaying a similar scheme for human melanoma.

A Review of Immunotherapeutic Strategies in Canine Malignant Melanoma

In dogs, melanomas are relatively common tumors and the most common form of oral malignancy. Biological behavior is highly variable, usually aggressive, and frequently metastatic, with reported survival times of three months for oral or mucosal melanomas in advanced disease stages. Classical clinical management remains challenging; thus, novel and more efficacious treatment strategies are needed. Evidence-based medicine supports the role of the immune system to treat neoplastic diseases. Besides, immunotherapy offers the possibility of a precise medicinal approach to treat cancer. In recent years, multiple immunotherapeutic strategies have been developed, and are now recognized as a pillar of treatment. In addition, dogs represent a good model for translational medicine purposes. This review will cover the most relevant immunotherapeutic strategies for the treatment of canine malignant melanoma, divided among five different categories, namely, monoclonal antibodies, nonspecific immunotherapy activated by bacteria, vaccines, gene therapy, and lymphokine-activated killer cell therapy.

Targeting macrophage anti-tumor activity to suppress melanoma progression

By phagocytosing cancer cells and their cellular debris, macrophages play a critical role in nonspecific defense (innate immunity) and, as antigen presenters, they help initiate specific defense mechanisms (adaptive immunity). Malignant melanoma is a lethal disease due to its aggressive capacity for metastasis and resistance to therapy. For decades, considerable effort has gone into development of an effective immunotherapy for treatment of metastatic melanoma. In this review, we focus on the anti-tumor activities of macrophages in melanoma and their potential as therapeutic targets in melanoma. Although macrophages can be re-educated through intercellular signaling to promote tumor survival owing to their plasticity, we expect that targeting the anti-tumor activity of macrophages remains a promising strategy for melanoma inhibition. The combination of tumoricidal macrophage activation and other treatments such as surgery, chemotherapy, and radiotherapy, may provide an effective and comprehensive anti-melanoma strategy.

Defining the Pharmacodynamic Profile and Therapeutic Index of NHS-IL12 Immunocytokine in Dogs with Malignant Melanoma

Background

Interleukin (IL)-12 is a pro-inflammatory cytokine that mediates T-helper type 1 responses and cytotoxic T-cell activation, contributing to its utility as anti-cancer agent. Systemic administration of IL-12 often results in unacceptable toxicity; therefore, strategies to direct delivery of IL-12 to tumors are under investigation. The objective of this study was to assist the preclinical development of NHS-IL12, an immunocytokine consisting of an antibody, which targets necrotic tumor regions, linked to IL-12. Specifically this study sought to evaluate the safety, serum pharmacokinetics, anti-tumor activity, and immune modulation of NHS-IL12 in dogs with naturally occurring cancers.

Methodology/Principal Findings

A rapid dose-escalation study of NHS-IL12 administered subcutaneously to dogs with melanoma was conducted through the Comparative Oncology Trials Consortium (COTC). Eleven dogs were enrolled in four dose-escalation cohorts; thereafter, an additional seven dogs were treated at the defined tolerable dose of 0.8 mg/m². The expanded cohort at this fixed dose (ten dogs in total) was accrued for further pharmacokinetics and pharmacodynamics assessment. NHS-IL12 levels, serum cytokine concentrations, and peripheral blood mononuclear cell characterization (post-treatment) and draining lymph node immune profiling, and tumor biopsies (pre- and post-treatment) were collected. Adverse events included thrombocytopenia, liver enzymopathies, fever, and vasculitis. Correlation between interferon (IFN)-γ induction, adverse events, and NHS-IL12 exposure (maximum concentration and area under the concentration-time curve) were dose-dependent. Serum IL-10 levels and intratumoral CD8⁺ populations increased after treatment. Partial responses, according to Response Evaluation Criteria in Solid Tumors (RECIST) criteria, were observed in two dogs treated with NHS-IL12 0.8 mg/m² and 1.6 mg/m².

Conclusions/Significance

NHS-IL12 was administered safely to dogs with melanoma and both immunologic and clinical activity was observed. This study successfully defined a narrow therapeutic window for systemic delivery of NHS-IL12 via the subcutaneous route. Results will inform the design and implementation of first-in-human clinical trials of NHS-IL12 in cancer patients.

Cytokine-Enhanced Vaccine and Interferon-β plus Suicide Gene Therapy as Surgery Adjuvant Treatments for Spontaneous Canine Melanoma

We present here a nonviral immunogene therapy trial for canine malignant melanoma, an aggressive disease displaying significant clinical and histopathological overlapping with human melanoma. As a surgery adjuvant approach, it comprised the co-injection of lipoplexes bearing herpes simplex virus thymidine kinase and canine interferon-β genes at the time of surgery, combined with the periodic administration of a subcutaneous genetic vaccine composed of tumor extracts and lipoplexes carrying the genes of human interleukin-2 and human granulocyte-macrophage colony-stimulating factor. Following complete surgery (CS), the combined treatment (CT) significantly raised the portion of local disease-free canine patients from 11% to 83% and distant metastases-free (M0) from 44% to 89%, as compared with surgery-only-treated controls (ST). Even after partial surgery (PS), CT better controlled the systemic disease (M0: 82%) than ST (M0: 48%). Moreover, compared with ST, CT caused a significant 7-fold (CS) and 4-fold (PS) rise of overall survival, and >17-fold (CS) and >13-fold (PS) rise of metastasis-free survival. The dramatic increase of PS metastasis-free survival (>1321 days) and CS recurrence- and metastasis-free survival (both >2251 days) demonstrated that CT was shifting a rapidly lethal disease into a chronic one. In conclusion, this surgery adjuvant CT was able of significantly delaying or preventing postsurgical recurrence and distant metastasis, increasing disease-free and overall survival, and maintaining the quality of life. The high number of canine patients involved in CT (301) and the extensive follow-up (>6 years) with minimal or absent toxicity warrant the long-term safety and efficacy of this treatment. This successful clinical outcome justifies attempting a similar scheme for human melanoma.

Immunotherapy for canine cancer--is it time to go back to the future?

Over the last 50 years, the significance of the immune system in the development and control of cancer has been much debated. However, recent discoveries provide evidence for a role of immunological mechanisms in the detection and destruction of cancer cells. Forty years ago veterinary oncologists were already investigating the feasibility of treating neoplasia by enhancing anticancer immunity. Unfortunately, this research was hindered by lack of a detailed understanding of cancer immunology, this limited the specificity and success of these early approaches. The great forward strides made in our understanding of onco-immunology in recent years have provided the impetus for a resurgence of interest in anticancer immunotherapy for canine patients. In this article both these initial trials and the exciting novel immunotherapeutics currently in development are reviewed.

Clinical trials of immunogene therapy for spontaneous tumors in companion animals

Despite the important progress obtained in the treatment of some pets' malignancies, new treatments need to be developed. Being critical in cancer control and progression, the immune system's appropriate modulation may provide effective therapeutic options. In this review we summarize the outcomes of published immunogene therapy veterinary clinical trials reported by many research centers. A variety of tumors such as canine melanoma, soft tissue sarcomas, osteosarcoma and lymphoma, feline fibrosarcoma, and equine melanoma were subjected to different treatment approaches. Both viral and mainly nonviral vectors were used to deliver gene products as cytokines, xenogeneic tumor associated antigens, specific ligands, and proapoptotic regulatory factors. In some cases autologous, allogenic, or xenogeneic transgenic cytokine producing cells were assayed. In general terms, minor or no adverse collateral effects appeared during this kind of therapies and treated patients usually displayed a better course of the disease (longer survival, delayed or suppressed recurrence or metastatic spread, and improvement of the quality of life). This suggests the utility of these methodologies as standard adjuvant treatments. The encouraging outcomes obtained in companion animals support their ready application in veterinary clinical oncology and serve as preclinical proof of concept and safety assay for future human gene therapy trials.

Immunotherapy in veterinary oncology

Tumor immunology and immunotherapy is one of the most exciting and rapidly expanding fields. The immune system is divided into 2 primary components: the innate immune response and the highly specific, but more slowly developing, adaptive or acquired immune response. Immune responses are separated by whether they are induced by exposure to a foreign antigen (active response) or transferred through serum or lymphocytes from an immunized individual (passive response). The ideal cancer immunotherapy agent should discriminate between cancer and normal cells (specificity), be potent enough to kill small or large numbers of tumor cells (sensitivity), and prevent recurrence of a tumor (durability).

Suicide plus immune gene therapy prevents post-surgical local relapse and increases overall survival in an aggressive mouse melanoma setting

In an aggressive B16-F10 murine melanoma model, we evaluated the effectiveness and antitumor mechanisms triggered by a surgery adjuvant treatment that combined a local suicide gene therapy (SG) with a subcutaneous genetic vaccine (Vx) composed of B16-F10 cell extracts and lipoplexes carrying the genes of human interleukin-2 and murine granulocyte and macrophage colony stimulating factor. Pre-surgical SG treatment, neither alone nor combined with Vx was able to slow down the fast evolution of this tumor. After surgery, both SG and SG + Vx treatments, significantly prevented (in 50% of mice) or delayed (in the remaining 50%) post-surgical recurrence, as well as significantly prolonged recurrence-free (SG and SG + Vx) and overall median survival (SG + Vx). The treatment induced the generation of a pseudocapsule wrapping and separating the tumor from surrounding host tissue. Both, SG and the subcutaneous Vx, induced this envelope that was absent in the control group. On the other hand, PET scan imaging of the SG + Vx group suggested the development of an effective systemic immunostimulation that enhanced (18)FDG accrual in the thymus, spleen and vertebral column. When combined with surgery, direct intralesional injection of suicide gene plus distal subcutaneous genetic vaccine displayed efficacy and systemic antitumor immune response without host toxicity. This suggests the potential value of the assayed approach for clinical purposes.

A snapshot of gene therapy in Latin America

Gene therapy attempts the insertion and expression of exogenous genetic material in cells for therapeutic purposes. Conceived in the 1960s, gene therapy reached its first clinical trial at the end of the 1980s and by December 2013 around 600 genuine open clinical trials of gene therapy were registered at NIH Clinical Trials Database. Here, we summarize the current efforts towards the development of gene therapy in Latin America. Our survey shows that the number of scientists involved in the development of gene therapy and DNA vaccines in Latin America is still very low. Higher levels of investment in this technology are necessary to boost the advancement of innovation and intellectual property in this field in a way that would ease both the social and financial burden of various medical conditions in Latin America.

Recent progress in canine tumor vaccination: potential applications for human tumor vaccines

Tumor vaccination holds great promise for the treatment of cancer and research concerning tumor vaccination in dogs is of great interest for veterinary as well as human medicine. Indeed, cancer is the leading cause of death in adult dogs and companion animals are acknowledged as excellent preclinical models for human oncology. The license of the veterinary melanoma vaccine (Oncept™) and Provenge® for the treatment of prostate cancer in men established tumor vaccination as a valid treatment modality for cancer. Although the results with this and other vaccines are promising, there are still some hurdles to overcome. In this article, preclinical and clinical trials with tumor vaccines in dogs are discussed, as well as the surplus value of canine cancer patients for human medicine.

Cationic lipid:DNA complexes allow bleomycin uptake by melanoma cells

Bleomycin is a chemotherapeutic agent barely diffusible through the plasmatic membrane. We evaluated DNA/cationic lipids complexes (lipoplexes) as mediators of its uptake in four spontaneous canine melanoma derived cell lines (Ak, Bk, Br and Rkb). Cell survival after lipofection plus or minus bleomycin was determined by the acid phosphatase method and the cellular uptake of lipoplexes, carrying the E. coli β-galactosidase gene, was evidenced by SYBR Green I staining. The four cell lines resulted sensitive to the bleomycin/lipoplexes system in both spatial configurations. Survival rates values were lower than 20% in monolayers of the four tested lines and lower than 30% in three lines (Ak, Bk and Rkb) when grown as spheroids. The sensitization to bleomycin depended on lipoplexes in Ak and Rkb while Bk (in both spatial configurations) and Br (as monolayers) were sensitive to bleomycin alone. Although some degree of sensitivity to bleomycin was induced by cationic lipids alone in Ak and Rkb monolayers, the maximal bleomycin effects appeared in the presence of lipoplexes. The sensitization was independent of transcriptional activity. The co-administration of lipoplexes diminished bleomycin IC50: 10-fold in Ak and Rkb monolayers; and sensitized the Ak and Rkb resistant spheroids. The bleomycin cytotoxic effects depended on lipoplexes concentration and diminished when cells were incubated at 8°C. Our results suggest that lipoplexes sensitize cells to bleomycin, increasing its uptake by an active transport mechanism, such as endocytosis. The bleomycin/lipoplexes system appears as a promising combination of chemotherapy and non-viral cancer gene therapy.

Cytokine-enhanced vaccine and suicide gene therapy as surgery adjuvant treatments for spontaneous canine melanoma: 9 years of follow-up

We present here the updated results after 9 years of the beginning of a trial on canine patients with malignant melanoma. This surgery adjuvant approach combined local suicide gene therapy with a subcutaneous vaccine composed by tumor cells extracts and xenogeneic cells producing human interleukin-2 and granulocyte-macrophage colony-stimulating factor. Toxicity was absent or minimal in all patients (0≤VCOG-CTCAE grade≤1). With respect to surgery-treated controls (ST), the complete surgery (CS) arm of this combined treatment (CT) significantly increased the fraction of local disease-free patients from 13 to 81% and distant metastases free from 32 to 84%. Even though less effective than the CS arm, the partial surgery (PS) arm of this CT was significantly better controlling the disease than only surgery (14% while PS-ST: 0%, P<0.01 and CS-ST: 5%, P<0.05). In addition, CT produced a significant sevenfold (CS) and threefold (PS) increase in overall survival. The CS-CT arm significantly improved both CS-ST metastasis-free- and melanoma overall survival from 99 days (respective ranges: 11-563 and 10-568) to >2848 days (81-2848 and 35-2848). Thus, more of 50% of our CT patients died of melanoma unrelated causes, transforming a lethal disease into a chronic one. Finally, surgery adjuvant CT delayed or prevented post-surgical recurrence and distant metastasis, significantly improved disease-free and overall survival maintaining the quality of life. Long-term safety and efficacy of this treatment are supported by the high number of CT patients (283) and extensive follow-up (>9 years). The successful clinical outcome encourages the further translation of similar approaches to human gene therapy trials.

Interferon-β lipofection I. Increased efficacy of chemotherapeutic drugs on human tumor cells derived monolayers and spheroids

We evaluated the effect of hIFNβ gene transfer alone or in combination with different antineoplastic drugs commonly used in cancer treatment. Five human tumor-derived cell lines were cultured as monolayers and spheroids. Four cell lines (Ewing sarcomas EW7 and COH, melanoma M8 and mammary carcinoma MCF-7) were sensitive to hIFNβ gene lipofection. Although this effect appeared in both culture configurations, spheroids showed a relative multicellular resistance (insensitive colon carcinoma HT-29 excluded). EW7 and M8 hIFNβ-expressing cells were exposed to different concentrations of bleomycin, bortezomib, carboplatin, doxorubicin, etoposide, methotrexate, paclitaxel and vincristine in both configuration models. In chemotherapy-sensitive EW7 monolayers, the combination of hIFNβ gene and antineoplastic drugs displayed only additive or counteractive (methotrexate) effects, suggesting that cytotoxic mechanisms triggered by hIFNβ gene lipofection could be saturating the signaling pathways. Conversely, in chemotherapy-resistant EW7 spheroids or M8 cells, the combination of hIFNβ with drugs that mainly operate at the genotoxic level (doxorubicin, methotrexate and paclitaxel) presented only additive effects. However, drugs that also increase pro-oxidant species can complement the antitumor efficacy of the hIFNβ gene and clearly caused potentiated effects (bleomycin, bortezomib, carboplatin, etoposide and vincristine). The great bystander effect induced by hIFNβ gene lipofection could be among the main causes of its effectiveness, because only 1 or 2% of EW7 or M8 hIFNβ-expressing cells killed more than 60 or 80% of cell population, respectively.

Cytokine-enhanced vaccine and interferon-β plus suicide gene as combined therapy for spontaneous canine sarcomas

Eleven soft tissue- and five osteosarcoma canine patients were subjected to: (i) periodic subcutaneous injection of irradiated xenogeneic cells secreting hGM-CSF and hIL-2 mixed with allogeneic or autologous tumor homogenates; and (ii) injections of cIFN-β and HSVtk-carrying lipoplexes and ganciclovir, marginal (after surgery) and/or intratumoral (in the case of partial tumor resection, local relapse or small surface tumors). This treatment alone (4 patients) or as surgery adjuvant (12 patients), was safe and well tolerated. In those patients presenting local disease (6/11), the suicide gene plus cIFN-β treatment induced local antitumor activity evidenced by the objective responses (3 complete, 2 partial) and stable diseases (2). In addition, the treatment prevented or delayed local relapse, regional metastases (lymph nodes developed only in 3/16) and distant metastases (0/16), suggesting a strong systemic antitumor immunity. The most encouraging result was the long survival times of 10 patients (>1 year, with good quality of life).

Genetically modified animals and pharmacological research

This chapter reviews the use of genetically modified animals and the increasingly detailed knowledge of the genomes of the domestic species. The different approaches to genetic modification are outlined as are the advantages and disadvantages of the techniques in different species. Genetically modified mice have been fundamental in understanding gene function and in generating affordable models of human disease although these are not without their drawbacks. Transgenic farm animals have been developed for nutritionally enhanced food, disease resistance and xenografting. Transgenic rabbits, goats, sheep and cows have been developed as living bioreactors producing potentially high value biopharmaceuticals, commonly referred to as "pharming". Domestic animals are also important as a target as well as for testing genetic-based therapies for both inherited and acquired disease. This latter field may be the most important of all, in the future development of novel therapies.

Cancer immunotherapy

The immune system is generally divided into 2 primary components: the innate immune response, and the highly specific but more slowly developing adaptive or acquired immune response. Immune responses can be further separated by whether they are induced by exposure to a foreign antigen (an "active" response) or whether they are transferred through serum or lymphocytes from an immunized individual (a "passive" response). The ideal cancer immunotherapy agent should be able to discriminate between cancer and normal cells (ie, specificity), be potent enough to kill small or large numbers of tumor cells (ie, sensitivity), and lastly be able to prevent recurrence of the tumor (ie, durability). Tumor immunology and immunotherapy is one of the most exciting and rapidly expanding fields at present.

Suicide gene therapy on spontaneous canine melanoma: correlations between in vivo tumors and their derived multicell spheroids in vitro

To validate the use of multicellular spheroids to predict the efficacy of herpes simplex thymidine kinase/ganciclovir (HSVtk/GCV) suicide gene therapy in the respective in vivo tumors, we established and characterized 15 melanoma-derived cell lines from surgically excised melanoma tumors. Three HSVtk-lipofected cell lines were not sensitive to GCV in any culture configuration, other five displayed similar sensitivity as monolayers or spheroids, and only one resulted more sensitive when grown as spheroids. Other six cell lines manifested a relative multicellular resistance (MCR) phenotype growing as spheroids, compared with the same cells growing as monolayers. The reverse correlation between the MCR and the monolayers survival to HSVtk/GCV suggests that one of the main causes of MCR would be the rapid cell repopulation after suicide gene treatment. The high correlation of MCR with the spheroids radial growth and with the mitotic index of the respective originary tumors supported this re-growth involvement. A remarkable finding was the high correlation in HSVtk/GCV sensitivity between in vivo tumor and the corresponding derived cell lines growing as spheroids (R(2) = 0.85). This strongly encourages the implementation of spheroids as highly realistic experimental model for optimizing and predicting the in vivo response of the respective tumors to therapeutic strategies.