Gene Therapy for Head and Neck Cancers
Gene Therapy for Head and Neck Cancers
ABSTRACT: Despite advances in surgery, radiotherapy, and chemotherapy, survival of patients with squamous cell carcinoma of the head and neck has not significantly improved over the past 30 years. Locally recurrent or refractory disease is particularly difficult to treat. Repeat surgical resection and/or radiotherapy are often not possible, and long-term results for salvage chemotherapy are poor. Recent advances in gene therapy have been applied to recurrent squamous cell carcinoma of the head and neck. Many of these techniques are now in clinical trials and have shown some efficacy. This article discusses the techniques employed in gene therapy and summarizes the ongoing protocols that are currently being evaluated in clinical trials. [ONCOLOGY 15(3):303-314, 2001]
Squamous cell carcinoma of the head and neck affects approximately 125,000 patients annually in developed countries worldwide. In the United States, an estimated 30,100 new cases will be identified in the year 2001, with 7,800 associated deaths. Primary therapy for localized disease is surgery with or without adjuvant radiotherapy or radiotherapy alone. Despite advances in surgical resection, reconstruction, and adjuvant treatment, survival in these patients has not improved significantly over the past 30 years.
Local and/or regional tumor recurrence develops in approximately one-third of patients following surgery.[3-6] In the majority of cases, disease recurs in the region of the original primary tumor and leads to severe morbidity due to pain, oropharyngeal dysfunction, and laryngeal obstruction, with resultant difficulties in swallowing and speech. Once the cancer has recurred and/or metastasized, the patient is often considered incurable. Conventional salvage treatment, including radiation therapy or surgery, is often difficult or disfiguring and offers little hope for long-term survival.
Several chemotherapeutic agents have been used in recurrent squamous cell carcinoma of the head and neck. Combination regimens with cisplatin (Platinol) and fluorouracil (5-FU) have induced responses in 32% to 47% of patients; however, this therapy can be toxic and shows no clear impact on survival.[7-12]
When recurrent squamous cell carcinoma of the head and neck becomes refractory to chemotherapy and/or radiation therapy, the median life expectancy is 3 months and the tumor response rate to second- or third-line chemotherapeutic agents is approximately 15%. Two-thirds of patients dying with this disease have no symptomatic distant metastases. Therefore, local and regional disease control is paramount, and there is an urgent need for more effective therapies for these terminally ill patients. Because of these considerations, much of the interest in treating squamous cell carcinoma of the head and neck currently lies in generating new and effective therapies such as gene therapy.
Gene therapy has the potential for targeting cancer cells while sparing normal tissues. These treatments are potentially useful for recurrent disease, as well as in the adjuvant setting (ie, at the resected margins). The purpose of this article is to describe the principles of gene therapy as they relate to head and neck cancer and to summarize the ongoing protocols that are currently being evaluated in clinical trials.
Gene therapy may be defined as the introduction of genetic material into a cell to modify cellular function. The transfer is either in vivo (in which the gene is introduced into the body) or ex vivo (in which a tumor is removed, the genetic material is delivered, and the cells are then reintroduced into the patient). The ex vivo approach has not been utilized in head and neck cancer because superficial lesions of the head and neck usually lend themselves to direct injection of genetic material.
Genetic material is transferred via vectors that may be chemical, physical, or viral. The ideal vector would transfer an exact amount of genetic material into a specific area of each target cell, thereby allowing proper expression of the gene product without causing toxicity. Unfortunately, the ideal vector does not exist.
Chemical transfection introduces DNA with calcium phosphate, lipid, or protein complexes. Lipid vectors are a combination of plasmid DNA (pDNA) and a lipid solution that result in the formation of a liposome. This fuses with the cell membranes of a variety of cell types, passing the pDNA into the cytoplasm and nucleus, where it is transiently expressed. DNA/protein complexes can be more specific than liposomes by conjugating DNA with a tissue-specific ligand or antibody. The DNA is thus internalized via receptor-mediated endocytosis. To prevent lysosomal DNA degradation, it is usually complexed to an endosome lysis agent.
Physical transfection of genes is accomplished by electroporation, microinjection, and ballistic particles. These have not been particularly useful in clinical trials as of yet. Currently, viral vectors are the most widely used method of genetic transfer.
Viruses in Gene Therapy
Viruses commonly used in gene therapies include retroviruses, adenoviruses, and herpesviruses.
Retroviruses: Retroviruses contain RNA genomes that undergo reverse transcription after infecting a cell, thereby producing double-stranded DNA. This DNA integrates in a stable, random fashion into the host genome, thus passing copies of the gene to all subsequent generations of cells. One limitation of retroviruses is that they can only infect actively dividing cells, leaving quiescent cells unaffected. As DNA is permanently inserted, this also raises long-term safety questions.
Adenoviruses: An adenovirus is a DNA virus that infects a cell, loses its protein coat, and transfers DNA into the nucleus, where it is transcribed. This DNA does not integrate into the host genome, and thus, its effects are transient (range: 7 to 42 days). Therefore, multiple administrations of the vector are usually required. The advantage of adenoviral vectors is that most cells are susceptible to infection, regardless of their position in the cell cycle. In addition, adenoviruses can be produced at a high titer—up to 1012 plaque-forming units—making their administration more efficient. As exposure to adenovirus is common, approximately 90% of humans have antibodies against the virus. Preexisting antibodies can limit the effectiveness of this strategy, particularly upon a second exposure to the vector.
Herpesviruses: Most herpesvirus vectors are developed from strains of herpes simplex virus type 1 (HSV-1). This is a double-stranded DNA virus that has several interesting properties, including the ability to remain latent in tissue and to be reactivated at the original site of infection. After infecting a cell, HSV-1 replicates within the cell, causing cell lysis and infection of surrounding cells. In addition, HSV-1 thymidine kinase (tk) is expressed during viral replication—a property that can be exploited to activate prodrugs, such as ganciclovir (Cytovene). In addition, HSV-1 is a common pathogen in humans and rarely causes significant illnesses. Table 1 summarizes the advantages and disadvantages of the various clinically significant vectors.