Download clinical advances in osteosarcoma immunotherapy

CLINICAL ADVANCES IN OSTEOSARCOMA IMMUNOTHERAPY;
IS THE CURE WITHIN REACH?
Nicola Mason BVetMed PhD DACVIM (Internal Medicine)
University of Pennsylvania
OSA is a highly aggressive mesenchymal bone tumor that occurs in middle to older aged large and giant
breed dogs and children in their first and second decades of life.1,2 It is estimated that OSA occurs with an
incidence of 10-15,000 dogs per year and approximately 800 children a year in the USA. It is the most common
primary bone tumor in dogs, affecting >10,000 dogs annually in the US and accounting for approximately 85%
of all skeletal tumors.3 Despite the relatively low number of children afflicted with this disease, it is the second
most common malignancy in adolescents 4. In both species, treatment consists of chemotherapy, radiotherapy
and radical surgery. Despite aggressive treatment, metastatic disease is common and results in mortality rates in
children of 30-40% within 5 years of diagnosis, making OSA the second highest cause of cancer-related death
in children and adolescents 5. In dogs, mortality rates are higher with 85-90% of dogs succumbing to metastatic
disease within 2 years despite standard of care treatment, which consists of amputation and follow up
chemotherapy. There are no effective treatments for metastatic disease in either children or dogs and the current
focus of drug development strategies is on prevention of metastases.6
Manipulation of the immune system to generate antigen-specific cytotoxic T cells that reduce tumor
burden and provide long-term protection against relapse is the holy grail of cancer immunotherapy. ASXS31164, a recombinant Listeria that expresses HER2/neu, presents a novel therapeutic approach that aims to
achieve these goals in both dogs and people with OSA.
OSA has long been considered an “immune responsive” tumor. In the late 1800’s, William Coley noted
that patients with bone sarcomas that suffered from concurrent Streptococcal infections, tended to have more
favorable prognoses than patients that were not infected. This observation lead to the development of Coley’s
toxins, a concoction of Streptococcus and Bacillus spp. that were administered regularly to patients with
sarcomas, including OSA, and caused potent, non-specific immune stimulation that lead to prolonged survival.7
Likewise, in more modern times, it has been recognized that both human and canine OSA patients live longer if
they develop bacterial infections at the surgical site post amputation, an effect that is presumably associated
with non-specific immune activation. 8 These observations lead to the development and use of bacterial cell wall
extracts (e.g. Liposome Encapsulated Muramyl Tripeptides (LE-MTPs)) that activate both innate and adaptive
immune responses and significantly prolong survival times in both species when administered following
standard of care amputation and chemotherapy. 9 Taken together, these findings indicate that an appropriately
activated immune system is likely to play an important role in controlling OSA. Much work is now being
performed to identify specific tumor targets in OSA (both primary and metastatic) that the immune system can
be trained to recognize and subsequently eliminate. One such tumor target is the epidermal growth factor
receptor HER2/neu.
HER2 is expressed in up to 60% of human and 40-80% of canine primary OSA samples as determined
by immunohistochemical analysis (10,11 and unpublished data). In human OSA patients, over-expression of
HER2 has been associated with a more aggressive phenotype, increased risk of metastasis and a significantly
worse histological response to pre-operative neo-adjuvant chemotherapy.12 Importantly, metastatic OSA also
expresses HER2/neu and recent studies indicate that targeting HER2/neu with antigen- specific T cells can
prevent metastatic disease in mice with OSA xenografts.13 Together these studies indicate that targeting an
immune response against HER2/neu may prevent metastatic disease and possibly delay the progression of
primary disease.
Listeria monocytogenes is a facultative aerobic intracellular bacteria that can readily infect antigenpresenting cells. Attenuated strains of Listeria, modified to express tumor antigens of interest have been used in
mouse models and in human patients to generation potent tumor specific cytotoxic T cell responses that lead to
tumor regression. The ability of Listeria to induce potent cytotoxic immune responses lies in its ability to
secrete a pore forming lysin (listeriolysin O) that allows it to escape the phagosome after internalization. Once
in the cytoplasm of the cell the Listeria and tumor antigens it carries can be processed and presented on the
surface of the antigen-presenting cell to stimulate cytotoxic T cells.
ADXS31-164 is a recombinant Listeria that expresses a chimeric human HER2/neu molecule.
Administration of this highly attenuated strain of bacteria to mice safely induces potent HER2/neu-specific
immune responses that lead to regression of established HER2/neu+ tumors and importantly, prevention of
HER2/neu+ metastatic disease.14 Two clinical trials have been performed to evaluate the use of ADXS31-164 in
dogs with appendicular OSA. In the first trial the hypothesis that administration of ADXS31-164 following
amputation and carboplatin chemotherapy could induce immune responses against HER2/neu and prevent
metastatic disease was tested. In the second trial, ADXS31-164 was combined with palliative radiation to
determine whether combination therapy could delay the progression of both primary and metastatic disease in
dogs with OSA that did not undergo amputation.
In the first trial it was found that intravenous administration of ADXS31-164 was safe and capable of
inducing immune responses against HER2/neu.15 Side effects that were observed were low grade and transient
and included mild fever, nausea and vomiting. These effects occurred within 2-4 hours of ADXS31-164
administration and subsided within several hours. The majority of dogs treated with ADXS31-164 developed T
cell responses against HER2/neu. Importantly, dogs that received ADXS31-164 had a statistically significant
improvement in overall survival when compared to a historical control group that received standard of care
amputation and chemotherapy. These results supported the hypothesis that the increase in overall survival in the
ADXS31-164 treated group was associated with the development of HER2/neu specific cytotoxic T cell
responses and the subsequent elimination of micrometastases.
In the second clinical trial with ADXS31-164 the synergistic therapeutic effects of combination
HER2/neu targeted immune therapy and radiation therapy (RT) for the treatment of primary HER2/neu+
appendicular OSA in dogs that do not undergo amputation was explored. Many of the dogs that develop OSA
are giant breed dogs, oftentimes with concurrent orthopedic problems that make them poor candidates for
amputation. Palliative radiation is frequently employed to reduce pain associated with the primary neoplasm.
However, ionizing radiation also induces immunogenic cell death, which promotes T cell responses against the
tumor. The trial tested the hypothesis that combining palliative radiation with ADXS31-164 would lead to more
potent T cell responses that may control the primary tumor and prevent metastatic disease. Dogs with a
histopathological diagnosis of OSA were enrolled. Patients received 2 consecutive doses of 8Gy palliative
radiation followed by 8 doses of ADXS31-164 given at 3-week intervals. Dogs were monitored for adverse
effects at the time of vaccination and throughout the course of the study. Side effects were low grade and
transient, mostly consisting of mild fever and vomiting. In a subset of dogs, combination therapy induced a state
of clinical and radiographic tumor dormancy leading to prolonged overall survival times. These preliminary
results need further exploration but they raise intriguing questions, not least of which is whether combination
radiation plus ADXS31-164 should be administered prior to amputation to generate HER2 specific immune
responses that may prevent gross metastatic disease that occurs in at least 25% of patients following amputation
and during their chemotherapy.16 The results of these trials will be discussed in more detail during the
presentation.
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