Methods
We retrospectively reviewed the Computerized Patient Record System for veterans with MM or metastatic CaP treated with ZA at the Memphis VAMC. Study inclusion criteria were aged > 18 years and care provided by a Memphis VAMC oncologist between January 2003 and January 2018. The study was approved by the Memphis VAMC’s Institutional Review Board, and procedures were followed in accordance with the ethical standards of its committee on human experimentation.
Using Microsoft SQL 2016 (Redmond, WA), we performed a query to identify patients who were prescribed ZA during the study period. Exclusion criteria were ZA prescribed for an indication other than MM or CaP (ie, osteoporosis) and receipt of ≤ 1 dose of ZA. Once a list was compiled, patients were stratified by ZA dosing interval: standard (mean, every month) or extended (mean, every 3 months). Patients whose ZA dosing interval was changed during treatment were included as independent data points in each group.
Skeletal-related events included fractures, spinal compression, irradiation, and surgery. Fractures and spinal compression were pertinent in the presence of radiographic documentation (eg, X-ray, magnetic resonance imaging scan) during the period the patient received ZA or within 1 dosing interval of the last recorded ZA dose. Irradiation was defined as documented application of radiation therapy to ≥ 1 bone sites for palliation of pain or as an intervention in the setting of spinal compression. Surgery was defined as any procedure performed to correct a fracture or spinal compression. Each SRE was counted as a single occurrence.
Osteonecrosis of the jaw was defined as radiographically documented necrosis of the mandible or associated structures with assessment by a VA dentist. Records from non-VA dental practices were not available for assessment. Documentation of dental assessment before the first dose of ZA and any assessments during treatment were recorded.
Medication use was assessed before and during ZA treatment. Number of ZA doses and reasons for any discontinuations were documented, as was concomitant use of calcium supplements, vitamin D supplements, calcitriol, paricalcitol, calcitonin, cinacalcet, and pamidronate.
The primary study outcome was observed difference in incidence of SREs between standard- and extended-interval dosing of ZA. Secondary outcomes included difference in incidence of ONJ as well as incidence of SREs and ONJ by disease subtype (MM, CaP).
Descriptive statistics were used to summarize demographic data and assess prespecified outcomes. Differences in rates of SREs and ONJ between dosing interval groups were analyzed with the Pearson χ2 test. The predetermined a priori level of significance was .05.
Results
Of the 300 patients prescribed ZA at the Memphis VAMC, 177 were excluded (96 for indication,78 for receiving only 1 dose of ZA, 3 for not receiving any doses of ZA). The remaining 123 patients were stratified into a standard-interval dosing group (121) and an extended-interval dosing group (35). Of the 123 patients, 33 received both standard- and extended-interval dosing of ZA over the course of the study period and were included discretely in each group for the duration of each dosing strategy.
