• Open Access

The Bone Biologic Effects of Zoledronate in Healthy Dogs and Dogs with Malignant Osteolysis

Authors


  • This study was conducted at the Veterinary Teaching Hospital, University of Illinois, Urbana, IL. Findings have been presented in part at the 26th Annual Veterinary Cancer Society Conference, Pine Mountain, GA, 2006.

Corresponding author: Timothy M. Fan, DVM, PhD, Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, 1008 West Hazelwood Drive, Urbana, IL 61802-4714; e-mail: t-fan@uiuc.edu.

Abstract

Background: Malignant osteolysis is a process whereby cancer cells in concert with osteoclasts erode bone matrix. Aminobisphosphonates (NBPs) such as zoledronate induce osteoclast apoptosis and thereby decrease malignant skeletal destruction, severity of bone pain, and frequency of pathologic fracture.

Hypothesis: IV-administered zoledronate will reduce homeostatic bone turnover in healthy dogs and pathologic bone resorption in dogs diagnosed with primary and secondary bone tumors.

Animals: Six healthy dogs and 20 dogs with naturally occurring primary or metastatic bone tumors were administered zoledronate IV.

Methods: Prospective study: In all dogs, healthy (n = 6) and with malignant osteolysis (n = 20), the bone biologic effects of zoledronate were evaluated by quantifying changes in serum C-telopeptide (CTx) or urine N-telopeptide (NTx) concentrations or both. In dogs with osteosarcoma (OSA) (n = 10), serial changes in tumor relative bone mineral density (rBMD) assessed by dual-energy x-ray absorptiometry were used to characterize zoledronate's antiresorptive effects within the immediate tumor microenvironment. Additionally, the biochemical tolerability of zoledronate was assessed in 9 dogs receiving multiple (≥2) consecutive treatments.

Results: All dogs had significant reductions in serum CTx or urine NTx concentrations or both after zoledronate administration. In a subset of dogs with appendicular OSA, reduced urine NTx concentrations and increased primary tumor rBMD coincided with improved limb usage as reported by pet owners in dogs treated with zoledronate and concurrent oral analgesics. Multiple zoledronate infusions were not associated with biochemical evidence of toxicosis.

Conclusions and Clinical Importance: In dogs with skeletal neoplasms, IV-administered zoledronate exerts bone biologic effects, appears safe, and can provide pain relief.

Dogs with bone-invasive tumors can initially present for pain. Neoplasms commonly associated with malignant osteolysis in dogs include appendicular osteosarcoma (OSA), multiple myeloma, and metastatic carcinomas arising from prostate, mammary, transitional cell, and apocrine gland anal sac tissues. Although various tumor types may cause discomfort, the mechanisms involved in generating and perpetuating bone cancer pain are universal. Within the bone tumor microenvironment, cancer cells dysregulate osteoclast activities, which in turn promote bone matrix erosion. Excessive bone resorption by osteoclasts and production of inflammatory peptides by tumor cells stimulate the nociceptor-rich endosteum and periosteum, creating sensations of pain.1

Bisphosphonates are drugs that specifically inhibit osteoclastic activity and are standard therapy for various disease conditions of bone resorption in human patients.2–5 Aminobisphosphonates (NBPs) such as pamidronate and zoledronate have increased antiresorptive potency from the addition of nitrogen to their molecular structure and are considered first-line treatments in people diagnosed with either multiple myeloma or skeletal carcinoma metastases. By inducing osteoclast apoptosis, NBPs significantly reduce the incidence and delay the time-to-onset of skeletal-related events, including pathologic fracture, spinal cord compression, and hypercalcemia.4,5 In addition to their potent antiresorptive effects, NBPs administered IV also demonstrate direct anticancer properties in vitro and in vivo and provide added rationale for their use in treating cancer-related bone disorders.6 Specifically in immortalized canine OSA cell lines, NBPs have been demonstrated to reduce cell viability and inhibit cell proliferation in a dose- and time-dependent manner.7–9

Assessing the effectiveness of NBPs in human cancer patients relies on subjective and objective measures. Self-reported decreases in bone pain can be a useful clinical indicator of therapeutic response to NBP therapy in people. However, more objective measures of NBP bone biologic effects require objective radiological and biochemical methodologies. Although cumbersome, serial imaging studies of affected skeletal sites with quantitative bone scintigraphy, computed tomographic scans, dual-energy x-ray absorptiometry (DEXA), and positron-emission tomography can provide objective measurements to corroborate clinical responses after antiresorptive NBPs therapy. However, given the need for specialized imaging equipment and facilities, biochemical surrogate markers of bone turnover have gained popularity for objectively monitoring response to NBPs therapy.10–13 Reductions in either serum or urine bone resorption markers reflect decreased osteoclastic activity and can serve as surrogate indices of effective antiresorptive therapy in human cancer patients receiving NBPs.11,12

Although both pamidronate and zoledronate provide symptomatic pain relief to human cancer patients by virtue of their antiresorptive effects, zoledronate has the advantage of being safely administered over 15 minutes in comparison with a recommended infusion time of 2–4 hours for pamidronate. In addition, zoledronate may also exert desirable direct and indirect antitumor activities, including apoptosis, antiangiogenesis, disruption of cell-to-cell interactions, and immunomodulation.7–9,14 Whereas recent studies have evaluated the use of pamidronate in dogs with malignant bone pain and shown promise with safe administration and some subjective and objective responses in cancer-bearing dogs,15–17 similar investigations using zoledronate are lacking. Because zoledronate is the preferred NBP for treating osteolytic complications in human cancer patients, investigation of its biologic effects in dogs with primary and secondary bone tumors is indicated. As such, the primary purpose of this study was to assess the bone biologic activity of zoledronate (0.25 mg/kg, IV) through the quantification of urine N-telopeptide (NTx) concentrations in healthy, skeletally mature dogs, and in dogs with primary and secondary tumors involving the skeleton. Because zoledronate has been rarely reported to induce renal tubular injury, a secondary endpoint of this investigation was to determine the biochemical tolerability of repetitive zoledronate infusions in tumor-bearing dogs.

Materials and Methods

Bone Biologic Effects of Single-Dose Zoledronate in Healthy Dogs

The antiresorptive potency and duration of effect for IV-administered zoledronatea (provided by Novartis Pharma) or 0.9% sodium chloride (sham-control) were evaluated in 2 cohorts of skeletally mature, clinician-owned dogs. Both cohorts of dogs were considered to be healthy based on history, physical examinations, and screening serum chemistry panels. Dogs of each cohort received IV infusions of either 0.25 mg/kg zoledronate (experimental, n = 6) or saline (sham-control, n = 6). The experimental cohort consisted of 6 neutered males (3 purebred and 3 mixed breed), with a median age and weight of 4.5 years (range 2.0–10.3) and 25.9 kg (range 14.6–34.8), respectively. The control cohort consisted of 4 neutered males and 2 spayed females (4 purebred and 2 mixed breed), with a median age and weight of 6.2 years (range 4.7–7.9) and 31.8 kg (range 23.2–35.3 kg), respectively. Based on a previous study, saline control and calculated doses of zoledronate (0.25 mg/kg) were reconstituted to a final volume of 100 mL in saline and administered as a 15-minute continuous rate infusion (CRI).18 Serial changes in urine NTx and serum C-telopeptide (CTx) concentrations were quantified weekly for 4 consecutive weeks to assess bone biologic activity changes in all dogs.

Malignant Osteolysis Study Population

Twenty dogs were prospectively evaluated between July 2004 and March 2007. Ten dogs (Group 1) presented for clinical lameness and radiographic focal osteolysis, and a definitive diagnosis of appendicular OSA was confirmed either by histopathology (n = 8) or by cytopathology and positive alkaline phosphatase staining (n = 2). In the remaining 10 dogs (Group 2), the presenting complaint varied depending on the type of primary tumor and extent (unifocal, multifocal, or diffuse) of skeletal involvement, but included generalized pain, restricted movement, polyuria/polydipsia, stranguria, and tenesmus. In Group 2 dogs, definitive diagnosis of the primary tumor was confirmed either by histopathology (n = 2) or by cytopathology (n = 8). Additionally, skeletal involvement was supported by one or more imaging modalities, including digital radiography (n = 10), bone scintigraphy (n = 3), and computed tomographic scans (n = 2). In Group 2, 7 dogs had definitive neoplastic skeletal involvement confirmed by premortem cytology (n = 4) or postmortem histopathology (n = 3). In the remaining 3 dogs, pathologic skeletal abnormalities were identified on digital radiographs, but evidence of neoplasia was not confirmed with either histopathology or cytology.

Bone Biologic Effects of Zoledronate in Study Dogs with Malignant Osteolysis

For the assessment of zoledronate's bone biologic effects, all dogs had to receive 1 dose of zoledronate administered IV (0.25 mg/kg), be reevaluated at least once after initial therapy, and not be treated with traditional cytotoxic therapies (chemotherapy and radiation therapy) within 14 days of initial zoledronate administration. Calculated dosages of zoledronate were diluted to a final volume of 100 mL with 0.9% sodium chloride and administered as a 15-minute CRI. Before the IV administration of zoledronate, pet owners were informed of available conventional treatment options, including surgery, adjuvant systemic chemotherapy, and palliative radiation therapy. Dogs were treated in accordance with the animal care guidelines of the University of Illinois Institutional Animal Care and Use Committee. All patients had baseline clinical staging, including complete blood count, serum chemistry panel, urinalysis, thoracic radiographs, and abdominal ultrasound. If not provided by referring veterinarians, digital radiographs of affected skeletal sites were also acquired. As dictated by the primary disease process, some dogs had additional diagnostics performed, including histologic or cytologic evaluation of bone marrow, spleen, lymph nodes, primary tumor or skeletal lesions or both, measurements of ionized calcium and parathyroid hormone, bone scintigraphy, and computed tomographic scans.

Group 1 Dogs. In addition to routine clinical staging, OSA-bearing dogs had urine collected in the morning for the quantification of NTx concentrations, DEXA scans to assess the primary tumor relative BMD (rBMD), and subjective limb usage reported by pet owners. Concurrently with zoledronate-administered IV, OSA-bearing dogs were treated with a standardized oral analgesic regimen that included deracoxibb (1–2 mg/kg once daily), tramadolc (2–4 mg/kg every 8 hours), and gabapentind (3–5 mg/kg every 12 hours). Serial reevaluations were performed at the University of Illinois Cancer Care Clinic every 28 days (median 28 days, range 25–31 days) and included physical examinations, serum biochemistry, urine NTx excretion quantification, rBMD calculation, pet owner perceived limb usage, and IV-administered zoledronate until clinical failure defined as pathologic fracture, advanced distant metastases, or inadequate pain control represented as progressive limb disuse reported by pet owners.

Group 2 Dogs. In addition to clinical staging, tumor-bearing dogs had urine collected in the morning for the quantification of NTx concentrations. Given the advanced stage of disease in some dogs, re-evaluations were performed at the University of Illinois Cancer Care Clinic 1 day after the first dose of zoledronate administered, then every 28 days (median 28 days, range 21–35 days). To accurately assess the immediate bone biologic effects of zoledronate, no other cytotoxic therapies such as systemic chemotherapy or radiation therapy were instituted for the skeletal lesions within a 24-hour period after the first zoledronate dose. If clinically indicated, all dogs were provided with supportive measures, including urinary catheter placement, IV saline diuresis, and oral analgesics including deracoxib (1–2 mg/kg once daily), tramadol (2–4 mg/kg every 8 hours), and gabapentin (3–5 mg/kg every 12 hours), irrespective of the timing of zoledronate administration. Reevaluations included repeat physical examination, serum biochemistry, assessment of urine NTx concentration, and IV-administered zoledronate until clinical failure, defined as end-stage local or metastatic tumor progression or both or unacceptable quality of life as determined by pet owners.

Determination of Urine NTx Concentrations

Urine samples (free-catch or cystocentesis) were immediately centrifuged at 4 °C, 450 ×g for 10 minutes, and the supernatant was collected and stored at −20 °C in 2-mL polypropylene cryovials until analysis was performed. Urine NTx concentrations were measured with a commercial ELISAe test kit, previously validated for use in the dog,19,20 and expressed as normalized nanomolar (nM) bone collagen equivalents (BCE) per millimolar (mM) concentration of urine creatinine.

Determination of Serum CTx Concentrations

Venous blood samples were collected via jugular venipuncture for the assessment of serum CTx concentrations. Whole-blood samples were centrifuged for 10 minutes at 450 ×g, and serum was separated and stored at −20 °C in 2-mL polypropylene cryovials until analysis. Serum CTx concentrations were measured by a commercially available immunoassay,f stated to be cross-reactive with canine serum.

Determination of Primary Tumor rBMD

At presentation and every subsequent visit, DEXAg scans were performed to measure BMD of the tumor and the equivalent anatomical area of the normal contralateral limb in Group 1 dogs. Three representative regions of tumor and normal limb were acquired and designated (T1, T2, and T3) and (N1, N2, and N3), respectively. rBMD of the primary tumor was calculated by the following formula:

image

Statistical Analysis

To assess the bone biologic activity of zoledronate in healthy, skeletally mature dogs, reductions in urine NTx and serum CTx concentrations in comparison with baseline values were evaluated with a repeated measures analysis of variance, with post hoc comparisons made with a Tukey-Kramer multiple comparisons test. To determine the individual bone biologic effects of zoledronate in tumor-bearing dogs, changes in urine NTx concentrations were analyzed with a paired t-test. To assess the cumulative bone biologic effects of zoledronate in tumor-bearing dogs, changes in urine NTx and/or rBMD were evaluated with repeated measures of variance, with post hoc comparisons made with a Tukey-Kramer multiple comparisons test. To determine biochemical tolerability as a function of cumulative zoledronate treatment cycles, serum parameters (creatinine, blood urea nitrogen [BUN], calcium, phosphorus, and potassium) representative of kidney function were evaluated with linear regression analysis. All values were reported as mean ± standard deviation. Statistical analysis was performed with commercial computer software.h Significance was defined as P < .05.

Results

Effects of Single-Dose Zoledronate in Healthy Dogs

Fluctuations in both urine NTx and serum CTx concentrations in dogs treated with saline control were not significant over time, P > .05 (Fig 1A,B). The IV administration of zoledronate (0.25 mg/kg) significantly reduced urine NTx and serum CTx concentrations in healthy dogs in comparison with baseline values on days 7, 14, 21, and 28 (P < .05 at all time points) (Fig 1C,D). Maximal average reductions in urine NTx (−77%) and serum CTx (−84%) concentrations from baseline were observed on days 21 and 14, respectively.

Figure 1.

 Biologic effect of zoledronate in skeletally mature dogs. Weekly quantification of urine N-telopeptide (NTx) and serum C-telopeptide (CTx) concentrations in 2 cohorts of healthy, skeletally mature dogs treated with either saline, IV (top panels, A and B) or zoledronate 0.25 mg/kg IV (bottom panels, C and D). Statistical differences in urine NTx and serum CTx concentrations in comparison with baseline values demonstrated by *. Significance defined as P < .05.

Effects of Zoledronate in Study Dogs with Malignant Osteolysis

For OSA-bearing dogs (Group 1, n = 10), the median number of zoledronate treatments was 3.5 (range 1–9). There were significant reductions in urine NTx concentrations (day 0: 322±187 nM BCE/mM creatinine versus day 28: 74.0±63.8 nM BCE/mM creatinine) after the first administration of zoledronate, P= .001 (Fig 2A). Furthermore, in 5 dogs with OSA and experiencing durable pain alleviation defined by >4 months of improved limb function reported by pet owners, there were significant (P < .05) and persistent reductions in urine NTx concentrations and increases in primary tumor rBMD in comparison with baseline values (Fig 2B,C). In dogs diagnosed with other tumor types besides OSA (Group 2, n = 10), the median number of zoledronate treatments was 1 (range 1–3). Tumor types represented in Group 2 dogs involving the skeleton included transitional cell carcinoma, apocrine gland anal sac adenocarcinoma, prostate carcinoma, multiple myeloma, plasma cell tumor, acute T cell leukemia, B cell lymphoma, histiocytic sarcoma, and giant cell tumor. Corroborating the observed bone biologic effects of zoledronate in OSA-bearing dogs, significant and immediate reductions in urine NTx concentrations (day 0: 634±370 nM BCE/mM creatinine versus day 1: 202±168 nM BCE/mM creatinine) were also demonstrated in dogs diagnosed with unifocal, multifocal, or diffuse tumors involving the skeleton, P= .003 (Fig 2D). When Groups 1 and 2 were combined, concentrations of NTx in urine were 510±326 nM BCE/mM creatinine and 150±143 nM BCE/mM creatinine before and after zoledronate administration, respectively, P < .001.

Figure 2.

 Changes in urine N-telopeptide (NTx) concentrations and relative bone mineral density (rBMD) in tumor-bearing dogs. For osteosarcoma-bearing dogs (n = 10), changes in (A) individualized urine NTx concentrations after the 1st initial zoledronate dose; and grouped (B) urine NTx concentrations or (C) rBMD after repetitive zoledronate treatments. For other tumor-bearing dogs (n = 10), changes in (D) individualized urine NTx concentrations after the 1st initial zoledronate dose. Significant differences from baseline values are indicated by *. Significance defined as P < .05. n, number of dogs at that time point.

Biochemical Toxicosis of Zoledronate in Study Population with Malignant Osteolysis

Biochemical evidence of toxicosis from IV-administered zoledronate was evaluated in 9 tumor-bearing dogs receiving ≥2 treatments. Serum variables, including creatinine, BUN, calcium, phosphorus, and potassium, were quantified as a function of cumulative zoledronate treatment cycles. There was no significant change in serum creatinine, calcium, phosphorus, or potassium concentrations with each successive administration of zoledronate. However, serum BUN concentrations significantly increased as a function of cumulative zoledronate treatment cycles, P= .01 (Table 1 and Fig 3).

Table 1.   Biochemical tolerability of zoledronate in dogs with malignant osteolysis.
ParameterZoledronate Treatment Cycle P-Value
Cycle (No. of dogs)Initial (n = 9)2 (n = 9)3 (n = 7)4 (n = 5)5+ (n = 5)
  1. BUN, blood urea nitrogen.

Creatinine (mg/dL)1.1 ± 0.21.2 ± 0.31.3 ± 0.31.3 ± 0.51.4 ± 0.5.08
BUN (mg/dL)15.5 ± 3.318.2 ± 5.920.8 ± 5.923.1 ± 8.022.2 ± 5.0.01
Calcium (mg/dL)11.0 ± 0.310.7 ± 0.510.5 ± 0.510.6 ± 0.411.0 ± 0.2.43
Phosphorus (mg/dL)4.4 ± 0.53.1 ± 0.83.2 ± 0.43.1 ± 0.74.1 ± 0.4.18
Potassium (mEq/dL)4.7 ± 0.64.3 ± 0.64.7 ± 0.44.5 ± 0.64.3 ± 0.6.30
Figure 3.

 Serum blood urea nitrogen as a function of zoledronate treatment cycle. Significant increase in serum blood urea nitrogen concentrations with a greater number of zoledronate treatments administered. All absolute serum blood urea nitrogen concentrations remain within normal reference range (7–31 mg/dL). Significance defined as P < .05.

Discussion

In this study, the IV administration of zoledronate to healthy dogs and dogs with skeletal cancers exerted bone biologic effects as demonstrated by reductions in circulating bone resorption markers. In 5 of 10 dogs with OSA, suppression of pathologic bone resorption was accompanied by subjective pain alleviation, resulting in improved limb usage and ambulation for >4 months. Additionally, the repeated IV administration of zoledronate to cancer-bearing dogs was well tolerated with no biochemical evidence of toxicosis being identified.

The pharmacokinetic and pharmacodynamic profile of zoledronate has been studied in human patients diagnosed with nonneoplastic and neoplastic bone disorders, and results from these investigations have established current recommended dosing regimens.3,21,22 Before justifying a specific dosing regimen in dogs, it is necessary to determine zoledronate's antiresorptive potency and duration of effect in dogs. In comparison with saline-treated dogs, the administration of zoledronate (0.25 mg/kg) significantly reduced urine NTx and serum CTx concentrations in comparison with baseline values for >28 days in healthy, skeletally mature dogs (Fig 1C and D). Based on these pharmacodynamic findings derived from a small number of animals, it appears that IV-administered zoledronate exerts potent and long-lasting bone biologic activity. These pilot findings in healthy dogs identify a biologically effective dose of zoledronate, which may be applied for the treatment of dogs with primary and secondary bone tumors.

Unlike traditional cytotoxic agents where therapeutic activities are substantiated by a measurable reduction in tumor burden, assessing the biologic effectiveness of NBPs for the management of malignant osteolysis is more difficult. Although self-reported decreases in bone pain can be a useful clinical indicator of therapeutic response to NBP therapy in humans, this subjective method of assessment is not possible for cancer-bearing dogs and cats. In human patients, objective measures for assessing response to NBPs include radiological and biochemical methodologies. Serial imaging studies of affected skeletal sites with quantitative bone scintigraphy, computed tomographic scans, DEXA, and positron-emission tomography, or the biochemical assessment of tumor-specific markers, bone-resorption and bone-formation indices, are accepted methodologies for quantifying response to NBP therapy.10–13 In this study, IV-administered zoledronate exerted potent bone biologic effects in dogs with malignant osteolysis as substantiated by significant reductions in urine NTx concentrations after zoledronate treatment. Additionally, in OSA-bearing dogs (n = 10), zoledronate's cumulative antiresorptive activity within the immediate bone tumor microenvironment was demonstrated by gradual but significant increases in rBMD as measured by DEXA scans on days 84 and 112 (Fig 2C).

Similar to other NBPs, the metabolic fate of zoledronate is either adsorption to bone matrix or elimination by renal excretion. Although considered safer than older generation NBPs such as pamidronate, acute tubular necrosis and azotemia after zoledronate administration has been documented in a small fraction of human patients.23–25 In 1 report describing adverse effects in 72 human patients, the average time to onset and number of zoledronate doses before renal deterioration was 56 days and 2.4 doses, respectively.25 In human cancer patients, predictive factors for developing zoledronate-induced renal dysfunction include patient age, cumulative number of doses, concomitant therapy with nonsteroidal anti-inflammatory drugs (NSAIDs), and current or prior treatment with cisplatin.26 In this study, a small number of dogs were treated with multiple doses of IV zoledronate, ranging from 2 to 9 (median 3.5). Serum variables representative of kidney function were assessed before each consecutive zoledronate infusion. Although no significant changes were identified in the majority of serum variables evaluated, serum BUN concentrations significantly increased as a function of cumulative zoledronate treatment cycles (Fig 3). One potential cause for the observed increase in serum BUN concentrations in these dogs is the long-term therapy with NSAIDs, with resultant subclinical gastrointestinal bleeding. Although serum BUN concentrations did gradually and significantly increase in this group of dogs, it should be emphasized that the absolute concentrations of serum BUN did not exceed the upper normal reference range (31 mg/dL) in any dog. Based on these collective findings derived from a small subset of cancer-bearing dogs, it would appear that repetitive zoledronate treatments are well tolerated and not associated with severe renal tubule necrosis and clinical azotemia. However, the rate of zoledronate-associated deterioration in renal function in humans is low (∼10–15%),27 and given the very limited sample size in the present study, the apparent renal tolerability of zoledronate in tumor-bearing dogs should be interpreted with caution, and continued serial renal function assessment would be prudent in dogs scheduled to receive repeated zoledronate treatments.

Although zoledronate administered IV has been definitively demonstrated to alleviate bone cancer pain and the frequency of pathologic fracture in human cancer patients,28,29 the current study was not designed to verify whether zoledronate exerts similar analgesic effects in dogs suffering from malignant bone pain. For such a determination, a double-blinded, placebo-controlled trial would be necessary to characterize the pain alleviating potential of zoledronate, used as a single-agent or adjuvantly, in cancer-bearing dogs. However, given that cancer-induced bone resorption is definitively linked with the generation of bone cancer pain,1,30,31 it is reasonable to believe that if zoledronate inhibits pathologic bone resorption in tumor-bearing dogs as demonstrated by biochemical and radiological methodologies, it would also have the potential of alleviating bone cancer pain. This supposition is bolstered by the durable pain alleviation defined to be >4 months of improved limb function reported by pet owners achieved in 5 OSA-bearing dogs treated with a combination of IV zoledronate and standardized oral analgesic therapy. To further corroborate the reported subjective pain alleviation as a consequence of reduced pathologic osteolysis, these same 5 OSA-bearing dogs also demonstrated prolonged suppression of urine NTx concentrations (Fig 2B) and gradual increases in tumor rBMD (Fig 2C).

Despite the possible therapeutic effectiveness of zoledronate in a subset of OSA-bearing dogs, its clinical utility for managing painful multifocal or diffuse skeletal lesions could not be determined in this study. Given the advanced metastatic nature or near-terminal tumor burden associated with some Group 2 dogs, most dogs were euthanized because of unacceptable quality-of-life scores as a direct result of progressive primary tumor growth and its clinical consequences, including rapid progression of neoplastic disease (leukemia), urinary outflow obstruction, and hyperviscosity. Additional investigations conducted in a more uniform population of cancer-bearing dogs with adequately controlled primary tumors will be required to verify the role of adjuvant zoledronate for treating secondary or metastatic tumors that involve the skeleton.

Although this study offers new information regarding the bone biologic effects of IV-administered zoledronate in dogs suffering from malignant osteolysis, several limitations should be addressed. First, although this investigation was prospective in nature, it was not designed to evaluate different zoledronate treatment regimens. Although the tested zoledronate dosing regimen (0.25 mg/kg IV as a 15-minute CRI every 28 days) demonstrated biologic activity consistent with inhibition of bone resorption, there is a strong possibility that lower dosages or greater dosing intervals would also exert meaningful biologic effects. Given the high cost of zoledronate, identifying the smallest dose and greatest dosing interval that is effective would be beneficial, especially because pamidronate, a relatively inexpensive NBP, alleviates pain in dogs with OSA16. Second, the sample population analyzed was small (n = 20) and heterogeneous with respect to many variables, including primary tumor type, extent of skeletal involvement, and concurrent supportive therapies. As such, in the OSA-bearing dogs, certain conclusions regarding long-term reductions in urine NTx concentrations and increases in rBMD were derived from a very limited number of patients (≤5) and should be cautiously interpreted. For the same limitations, conclusions regarding the long-term biochemical tolerability of zoledronate should be tempered by the fact that only a small number of dogs were analyzed (≤5). Because the incidence of zoledronate-induced biochemical derangements, including hypocalcemia, hypophosphatemia, hypokalemia, and azotemia, observed in human cancer patients is relatively low (<15%),23,24,26,32–34 the small population size in this canine study is underpowered to detect the true incidence of zoledronate toxicosis. Third, given the investigational nature and undocumented efficacy of zoledronate for managing osteolytic pain in dogs, the authors felt ethically bound to treat tumor-bearing dogs with not only zoledronate but also conventional oral analgesic drugs. Given the concurrent administration of zoledronate and other analgesic drugs, it is not possible to verify the sole clinical effects of zoledronate for managing malignant osteolytic bone pain in tumor-bearing dogs. A randomized, double-blinded, placebo-controlled study would have to be conducted to verify the clinical benefit that is afforded to dogs with bone tumors treated with conventional palliative therapies and adjuvant zoledronate.

Despite the many limitations of this study, it solidly demonstrates that IV-administered zoledronate exerts potent biologic effects in bone tumor-bearing dogs, as reflected by rapid, significant, and sustained reductions in urine NTx concentrations. Given that malignant osteolysis is tightly correlated with the genesis of bone cancer pain, strategies that inhibit pathologic skeletal destruction are likely to be of benefit in palliating dogs diagnosed with bone neoplasms. This investigation provides key information to use in rationally designing prospective studies evaluating the therapeutic utility of zoledronate for managing cancer pain associated with canine neoplastic bone conditions.

Footnotes

aZometa, Novartis Pharma, East Hanover, NJ

bDeramaxx, Novartis Animal Health, Greensboro, NC

cUltram, Ortho-McNeil Inc, Raritan, NJ

dNeurontin, Pfizer, New York, NY

eOsteomark NTx urine, Ostex International Inc, Seattle, WA

fSerum Crosslaps, Nordic Bioscience, Herlev, Denmark

gQDR-4500W, Hologic, Bedford, MA

hGraphPad InStat, GraphPad Software Inc, San Diego, CA

Acknowledgments

The authors thank Drs David Heller, Lorin Hillman, Jackie Wypij, Pamela Lucas, Virginia Coyle, and Jennifer Marretta and Mrs Nancy George, Jenny Rose, and Rebecca Moss of the Cancer Care Clinic for patient management.

Ancillary