Osteosarcoma: new tricks for old dogs (Proceedings)

This talk will cover a general review of osteosarcoma then discuss various novel therapies currently available for the local and systemic treatment of this cancer in dogs. Appendicular osteosarcoma (OSA) will be the focus of the talk, but axial skeleton OSA will be covered where appropriate. OSA is a devastating disease in dogs. Because tumors often occur in the weight-bearing skeleton, dogs are usually presented for severe pain and lameness. Bones affected with OSA can fracture with seemingly minor activity, especially activity involving torque on the bone without concurrent full weight bearing. Because these tumors typically occur near joints, and in weight-bearing locations, it is usually not possible to spare the limb without destroying the function of adjacent joints, necessitating amputation in most cases. Distal radial OSA can be treated with various methods of surgical limb salvage because the carpus can be arthrodesed. OSA is highly metastatic and 90% of affected dogs are assumed to have metastatic disease at the time of diagnosis, though metastatic disease is only found in 10-15% of cases initially. Amputation alone results in an average survival of only 3-5 months, whereas amputation followed by chemotherapy increases the median survival to 8-12 months.(Watson 2002)

Axial skeleton OSA is locally aggressive and median survival times commonly fall around 4 months. Failure is local and therefore the full metastatic potential is not appreciated, however it is thought that OSA of the flat bones of the skull has a lower metastatic behavior than other skeletal locations. The mandible has been reported to have a metastatic rate of approximately 30%. Local therapies include surgery and radiation therapy, as well as radioactive, bone-seeking therapies such as samarium-153-EDTMP.

Osteosarcoma is an excellent model for the same cancer in people. Molecular abnormalities are similar and clinical progression of the disease as well as response to therapy are similar between dogs and people.(Krishnan 2006) Dogs share our environment, experience similar genetic selection pressures, are an outbred species that spontaneously develops cancer, and have an intact immune system, unlike laboratory animal models (rodents). In addition, dogs live naturally shorter lives and thus the impact of new cancer treatments can be evaluated in significantly less time than in studies in people.

Staging

Preliminary staging includes bloodwork and urinalysis, as well as radiographs of the affected bone/limb and the lungs. Since 90% of osteosarcoma is considered to be at least micrometastatic (with 10-15% of those presenting with macroscopic, radiographically evident metastasis), thoracic radiography is an important step. Bloodwork may reveal an elevated alkaline phosphatase (ALP) and is essential for ensuring the health of the patient, as systemic chemotherapy is anticipated. Most chemotherapy regimens are platinum-based, and so the renal assessment is of particular interest, as platinum agents can be nephrotoxic (eg. cisplatin).

Radiographic evaluation of the bone lesion is of critical importance. Hallmark radiographic signs include: combination of lysis and proliferation, loss of trabecular pattern to the marrow with an indistinct transition between normal and abnormal bone, extension to the soft tissues with mineralization (often in a "sunburst" pattern), metaphyseal location, limited to one side of the joint, and endosteal destruction/cortical thinning. These changes are so consistent with osteosarcoma radiographs are sometimes more reliable at determining the diagnosis than biopsy. Although biopsies can be performed, and may afford peace of mind to the owner prior to amputation, a small bone biopsy may reveal reactive bone that clouds a definitive diagnosis and the biopsy tract can create a weak point prone to pathologic fracture.

To minimize the risk of biopsy-induced pathologic fracture while still confirming the diagnosis, cytology has been increasingly used in combination with staining of cytology specimens for ALP. ALP staining on cytology is highly sensitive (up to 100% positive with true OSA).[4] If a bone tumor is mesenchymal in origin with cytology characteristic of OSA and is ALP positive, it is almost certainly OSA. Additional staging tests include nuclear scintigraphy to screen for second bony sites of disease. Secondary sites that are in unusual locations and not consistent with arthritis or other benign processes, are suspicious for second OSA sites and occur in approximately 5% of cases.(Jankowski 2003) Finally, the regional lymph node should be evaluated, which is good practice for any tumor type. Though OSA spreads almost exclusively hematogenously, lymph node involvement is found in approximately 4% of dogs and warrants a very poor prognosis, with a median disease-free interval of only 48 days in dogs with positive nodes vs. 238 days without.(Hillers 2005)

Prognostic factors

In addition to the intuitive negative effects of advanced stage on disease, there are a few interesting prognostic factors with canine OSA. If ALP is elevated at the time of diagnosis, the prognosis is shorter than for dogs with normal ALP (median survival time of 5.5 vs. 12.5, respectively, Garzotto 2000), and prognosis is worse for dogs whose ALP does not decrease following amputation (Ehrhart 1998). Reasons for this are unclear but this has been true in several studies in dogs, and has also been shown in people. Also, though not commonly applied by pathologists, grade is prognostic. This is logical and certainly holds true for most tumors, however since 75% of OSA are grade 3, and less than 5% are grade 1, it is not often clinically useful to apply a grading scheme. It should be recognized, however that the occasional low grade OSA may have a better prognosis than most cases. (Kirpenstein 2002) Finally, a very interesting observation by the Colorado State University Animal Cancer Center is that of their allograft limb salvage patients, those that develop a chronic infection live twice as long as those that do not. Explanations for this are currently under investigation.

Molecular advances

Ezrin

Ezrin is a protein responsible for linking the F-actin in the cytoskeleton to the cell membrane for motility. Disregulation and alterations in expression have been seen in human and canine osteosarcoma. Specifically, ezrin appears to play an important role in metastasis of osteosarcoma. Even more importantly, ezrin is preferentially expressed at the "leading edge" or the "invasive front" of the tumor specifically, suggesting a role in progression of osteosarcoma metastasis. Furthermore, protein kinase C is involved in phosphorylation of ezrin and its co-functional molecules. These findings help us further understand the biology of metastasis and offer novel therapeutic targets. (Ren 2009)

P53

This tumor suppressor gene is commonly mutated in human cancers. For dogs with appendicular OSA, mutated p53 occurs in 40% of cases and warrants a poorer prognosis (median survival time 81 days vs. wild type p53 at 256 days). (Kirpenstein 2008) p53 is overexpressed in canine OSA and can vary by breed and histologic subtype.

Standard treatment options

Local therapies

Standard of care treatment for most dogs with osteosarcoma is amputation of the affected limb. Most statistics of outcome have been reported following amputation with or without adjuvant chemotherapy. Additionally, attempts to spare the affected limb have included autograft, allograft, and prosthetic (metal implant) limb sparing surgeries. Pioneered at the Colorado State University Animal Cancer Center, allograft replacement has served as a prototype for this procedure in people. Since the advent of limb salvage surgery, various surgeons and oncologists have developed alternative methods for sparing an affected limb. In some dogs this is primarily cosmetic. Although they could function well without a limb, the pet owner feels that the dog's quality of life will be optimal if function of the affected limb is preserved. In other cases, dogs that are crippled with arthritis, are already missing another limb, or other reasons, limb salvage may be necessary.

Novel methods of surgical limb salvage include distraction osteogenesis and ulnar transposition. Locations such as the ulna and the scapula can be removed while maintaining function of the limb, without a need to replace the missing bone.

Medical/radiotherapy limb salvage has also been pursued using the bone-seeking radioisotope samarium, palliative pain control (without definitive intent for the limb but to allow maintenance of the limb for as long as possible), or external beam radiation therapy.

Systemic therapies

While most dogs (and people) can compensate for the loss of a limb, systemic spread of primary appendicular OSA is most often the life-limiting event. Though axial skeleton OSA is often limited by local disease, metastatic potential must be considered as local therapies evolve and improve. Chemotherapy is the mainstay of treatment for delaying the development of systemic metastasis. With adequate local control followed by chemotherapy, approximately half the remaining dogs survive each year with 1, 2, and 3 year survival rates of approximately 50%, 25%, and 10%.

Novel treatment options

Local therapies

In addition to continued development of novel surgical limb salvage methods, the University of Missouri has been developing intralesional limb salvage techniques using novel radiopharmaceutical agents. This approach is ideal for smaller lesions where dosimetry can be optimized. A few dogs have experienced durable tumor control with preservation of limb function and excellent quality of life. Although this involves a surgical approach for injection, recovery is rapid and no myelosuppression has been seen, which is a significant limitation of samarium therapy.

Systemic therapies

Standard chemotherapy continues to improve survival, but new options may increase convenience or tolerability of treatment, or may offer the hope of a disease-free and survival time that is longer than past reports.

The novel platinum drug satraplatin is the first orally bioavailable platinum chemotherapy agent. At the University of Missouri we have recently completed a phase 1 dose-finding study and as part of that study, 4/6 dogs treated with osteosarcoma in the adjuvant setting following amputation were disease-free for more than one year. Of the 4, one was lost to follow up at 1 year, one developed metastasis at approximately 450 days, and two more continue to be disease free at greater than 430 and 480 days. Capsules were easily administered and gastrointestinal toxicity was mild, with myelosuppression as the dose-limiting toxicity.

"All that's old is new again." Immunotherapy using liposome-encapsulated muramyl tripeptide was investigated in the 1980's and results of trials in dogs of improved survival times lead to approval for use in children with osteosarcoma. This immunomodulatory agent may resurface as a possible treatment option in dogs with osteosarcoma.

Treatment of pulmonary metastasis

Historically, no treatment is effective at treating pulmonary metastatic osteosarcoma. However, novel therapies include rapamycin and inhalant chemotherapy. Rapamycin is the "father" of a group of drugs called rapalogs which inhibit the m-TOR pathway (mammalian Target Of Rapamycin). This pathway is primarily involved in sensing the nutrient conditions for a cell. Preclinical studies have shown activity of rapamycin in treating metastatic pulmonary osteosarcoma. Clinical trials in tumor-bearing dogs are currently ongoing through the Comparative Oncology Trials Consortium, of which the University of Missouri is a member.

Metronomic chemotherapy involves small, daily or every other day doses of chemotherapy, often with alkylating agents. Satraplatin (described above) is currently under investigation for metronomic use. Metronomic chemotherapy aims to stop or slow angiogenesis to a growing tumor, thereby eliminating nutrients for and stopping tumor growth. Satraplatin caused regression of a metastatic pulmonary osteosarcoma lesion in one dog, and is now under investigation for metronomic chemotherapy.

The author has focused recently on detection and treatment of primary and metastatic lung cancer, and in this research has used inhalant therapies. By delivering therapy by airway, we are able to get higher concentrations of the therapy, such as cytotoxic chemotherapy or immunotherapy directly to the lungs as a target organ, avoiding first-pass metabolism and minimizing or completely avoiding systemic toxicity. Lower doses of therapeutic agents can be delivered with increased opportunity for beneficial effect. Delivery methods include mask inhalation and endotracheal instillation via mesh nebulization or catheter delivery. Agents that have been used include liposomal IL-2 (causing durable regression of metastatic pulmonary osteosarcoma in 2/4 dogs), 5-fluorouracil, doxorubicin, paclitaxel, and in the author's laboratory, cisplatin and gemcitabine. Though investigations are ongoing, preliminary investigations show promise and new technology offers improved methods for aerosol delivery.

References

Watson CL, Lucroy MD. Primary appendicular bone tumors in dogs. Compendium on Continuing Education for the Practicing Veterinarian 2002; 24:128-38, 147.

Krishnan K, Khanna C, Helman LJ. The molecular biology of pulmonary metastasis. Thoracic Surgery Clinics 2006; 16:115-24.

Hillers KR, Dernell WS, Lafferty MH, et al. Incidence and prognostic importance of lymph node metastases in dogs with appendicular osteosarcoma: 228 cases (1986-2003). J Am Vet Med Assoc. April 2005;226(8):1364-7.

Garzotto CK, Berg J, Hoffmann WE, Rand WM. Prognostic significance of serum alkaline phosphatase activity in canine appendicular osteosarcoma. J Vet Intern Med. 2000 Nov-Dec;14(6):587-92.

Kirpensteijn J, Kik M, Rutteman GR, Teske E. Prognostic significance of a new histologic grading system for canine osteosarcoma. Vet Pathol. March 2002;39(2):240-6.

Kirpensteijn J. TP53 gene mutations in canine osteosarcoma. Vet Surg. July 2008;37(5):454-60.

Ehrhart N, Dernell WS, Hoffmann WE, et al. Prognostic importance of alkaline phosphatase activity in serum from dogs with appendicular osteosarcoma: 75 cases (1990-1996). J Am Vet Med Assoc. October 1998;213(7):1002-6.

Jankowski MK, Steyn PF, Lana SE, et al. Nuclear Scanning with 99mTc-HDP for the Initial Evaluation of Osseous Metastasis in Canine Osteosarcoma. Vet Comp Oncol. September 2003;1(3):152-158.

Ren L, Hong SH, Cassavaugh J, et al. The actin-cytoskeleton linker protein ezrin is regulated during osteosarcoma metastasis by PKC. Oncogene. 28(6):792-802, 2009 Feb 12.