Live Viruses in Cancer Treatment
Live Viruses in Cancer Treatment
Most people think of
"viral therapy" as an obscure, experimental approach to the treatment
of disease. However, replicating viruses have been used as an effective
therapeutic modality for more than 200 years. One of the greatest clinical
advances in medical historythe eradication of smallpoxwas made with a
replicating virus. In 1796, Edward Jenner discovered that pus from the wounds of
infected patients contained live cowpox virus, which could be used as an
effective vaccine against smallpox. This discovery lead to the vaccination of
several million people and world clearance of the disease.
Interestingly, rare reports of complete remission induced in cancer patients
in association with smallpox vaccination were sporadically reported.
Observations of tumor regression in association with other viral infections have
also been described in cancer patients infected with herpes zoster,[4,5]
hepatitis virus,[6,7] influenza, varicella, measles,[10-12] and other
viruses.[13-15] The first published report of tumor destruction related to
replication competent viruses occurred in 1912 when a woman with cervical cancer
developed significant tumor necrosis following administration of an attenuated
rabies virus for prophylactic treatment after a dog bite.[16,17] Early in vitro
demonstration of viral oncolysis was first shown in 1922 with vaccinia virus,
which was shown to propagate in several malignant tumor lines.
Following these observations, extensive work was performed investigating the
potential use of viral therapy to treat cancer. In 1950, a strain of
encephalitis virus was shown to induce a dose-related oncolytic effect in vivo
with a mouse sarcoma tumor. Viral replication was shown to correlate with
tumor cell lysis; however, viral encephalitis developed in several mice. It was
later found that serial passaging of the encephalitis virus in vitro prior to
tumor injection in vivo would reduce the proliferative capacity in normal
tissues, thereby minimizing the occurrence of encephalitis and enhancing
oncolytic capacity. Clinical investigation was stimulated following additional
work with Newcastle disease virus (NDV), influenza virus, and other viruses in
animal models that showed cessation of ascites tumor (Ehrlich cells) growth and
eradication of the malignancy in some animals.[19-23]
Clinical trials were carried out in advanced cancer patients between 1950 and
the early 1970s, investigating administration of replication-proficient
viruses.[17,24-29] Transient responses were seen. Several mechanisms of action
were described, involving direct tumor lysis related to viral proliferation,
tumor antigen induced immune activation, modulation of cancer oncogene
expression (ie, c-fos, protein kinase C [PKC] modulation with measles
injection), apoptosis related to expression of unique viral proteins (ie,
E1A),[30,31] release of immunostimulatory cytokines,[32,33] and activation of
other antitumor immune responses (ie, natural killer [NK] cell activation).
Viruses with low pathogenicity for normal tissue and high oncolytic capacity
were investigated. Such viruses include NDV, mumps virus, herpes simplex virus (HSV),
Egypt 101 virus, influenza virus, adenovirus serotype 5, vaccinia, and ONYX-015.
Historical and current studies will be discussed.
Most oncolytic viruses require proliferation in the same species or cell
lineage and depend on host factors for successful evolution through life-cycle
stages (binding, entry, intracellular transport, genome replication, viral gene
expression, assembly and release of progeny). To initiate this process, the
virus requires a suitable receptor at the surface of the cell for uptake and
transcription factors to bind to the promoter/enhancer elements in the viral
genome, to induce expression of viral DNA.
Manipulations of the viral coat protein genes and tumor-specific viral
promoters have not adversely affected replication of oncolytic viruses in
malignant tissue, but have limited replication capacity in normal tissue.
Additional specificity to malignant tissue has been shown following modification
of the viral coat protein thereby enabling specific binding to tumor antigens
not expressed on normal cell surfaces, and engineering of tumor specific
promoter and enhancer regions with the viral genome to generate viruses with
selective malignant cell replication capacity.[37,38]
The release of oncolytic virus progeny (up to 104 viruses per cell) coincides
with the death of the host tumor cell. The first "burst" of
replicating viruses generally occurs less than 24 hours after treatment and
may continue as long as conditions are favorable for replication and immune
destruction of released virus is limited. Malignant cells are capable of evading
immune defenses, and this effect may facilitate local spread of released
Egypt 101 virus is a strain of the West Nile virus, which is an adenovirus
subtype. Preclinical testing of Egypt 101 virus in the early 1950s showed
oncolytic activity in a uterine/cervix cancer cell line (HeLa). Testing of
live virus administration via oral and intravenous routes in normal volunteers
revealed minimal toxicity (low-grade fever), thereby justifying clinical trials
in cancer patients,[42-44] although a small number of patients with hematologic
malignancies did develop transient encephalitis. Fever generally occurred
within 48 hours after inoculation and often coincided with the detection of live
virus in circulation or excretion.
In the first such trial, involving 34 cancer patients (27 evaluable for
response), tumor regression was observed in 4 patients, stabilization occurred
in 5, and 18 showed no response to a single injection of live virus. A
subsequent trial involving 30 patients with cervical carcinoma tested several
routes of inoculation (direct intratumoral injection, arterial infusion,
intravenous infusion).[43,45] Toxicity was limited to low-grade fever, and
regression or stabilization of disease was observed in the majority of patients.
Unfortunately, most responses were transient (< 3 months), and no patients
achieved a durable complete response.
Analysis of cervical tissue and vaginal smears revealed proliferating virus
in 77 samples from 20 patients studied. However, with analysis of 140 samples,
10 patients showed no evidence of viral presence. Response did not necessarily
correlate with recovery of virus, although patients achieving more extensive
necrosis generally harbored detectable virus. Studies to explore more intensive
dosing of the virus or combination with other anticancer agents were not
Mumps, a paramyxovirus, has a tight helical RNA inner core enclosed in an
outer lipid/protein shell. Oncolytic efficacy of mumps virus was initially
demonstrated in a rat sarcoma model.
The first clinical trial investigating mumps virus involved 90 patients with
advanced cancer and explored several routes of administration including oral,
rectal, intratumor, inhalation, and intravenous, depending on the location of
the tumor. Initial hematologic response to treatment included leukocytosis
followed by lymphopenia. Transient fever, which could be inhibited by
prophylactic treatment with low-dose prednisone, was also observed.
The authors noted that elevated antibodies at baseline were associated with a
lesser tumor response. However, three of the four patients achieving an
"optimal" response had elevated neutralizing antibodies to mumps virus
prior to treatment. Overall, 37 (41%) patients achieved a ³
50% reduction in tumor size, and 79 patients with stable disease or
better demonstrated clinical improvement (improved appetite, reduced pain,
increased body weight).
Of the 90 patients, 65 received a combination of local intratumoral injection
and/or intravenous infusion of the virus, and 24 (37%) of these 65 patients
showed a partial or complete response. Most partial or complete responses
occurred in patients with gastric carcinoma. However, the highest proportion of
complete or partial responses occurred in cutaneous carcinoma and uterine
carcinoma. Additionally, 9 of 10 patients with metastatic pulmonary carcinoma
achieved clinical improvement with regression in tumor bulk.
Patients receiving multiple intratumoral injections over a prolonged period
achieved a higher response rate and longer duration of response.[46,47] Fifteen
patients received intravenous mumps virus alone. Six of these patients received
fewer than nine intravenous treatments, and none had a positive response. In
contrast, of the nine patients receiving nine or more systemic treatments, six
achieved a response (P < .02). This was the first study to suggest that a
multitreatment administration schedule may have a clinical advantage.
Further exploration of a systemic route of administration was not performed.
A follow-up study involving 200 cancer patients administered mumps virus
intratumorally. Toxicity was minimal. Transient tumor regression was noted
in 26 patients. Responses were observed in patients with cancer of the breast,
rectum, colon, thyroid gland, uterus, and skin. Due to the transient nature of
the response and the difficulty in manufacturing a uniform product, further
clinical testing of the mumps virus was not pursued.