Bone strength and surrogate markers: The first, second, and third fiddle

Ever since the publication of the United States fluoride clinical trial data showing that the studied dose of fluoride therapy in postmenopausal osteoporosis (PMO) induced a linear increase in spinal bone mineral density (BMD) but without a reduction in fracture risk, registration for all subsequent therapies for the treatment of PMO required evidence for fracture risk reduction as the primary endpoint—the first fiddle. The previous preemptive role that this surrogate marker (pharmacologically-induced increase in BMD) had held as the indicator for improvement in bone strength now became the second fiddle. In a symphony orchestra the second fiddle plays a less robust role to the first fiddle—changes in BMD became the less robust endpoint for risk reduction and it now became the second fiddle for registration of therapies for the treatment of PMO. Nevertheless, despite fracture reduction required for primary registration, the subsequent registration of the intermittent dosing formulations of bisphosphonate therapies for the treatment of PMO, with the exception of annual intravenous (IV) zoledronic acid, were approved, not on the basis of any fracture data, but on the demonstration that the weekly or monthly oral, or quarterly IV bisphosphonates showed a noninferior increase in lumbar spine BMD as the fracture-proven daily formulation. For these studies, the second fiddle regained the first fiddle’s chair. Annual zoledronic acid registration for PMO still required fracture data for primary registration, because there had not been a previous dosing formulation studied with a placebo group in a fracture trial with this bisphosphonate. In the years 2000 and 2002 there appeared two meta-analyses from similar datasets, examining the relationship between osteoporosis drug–induced increases in spine BMD and fracture risk reduction that came to dissimilar conclusions: the first, that the relationship between increases in BMD and risk reduction was linear; the second, that it was not. Since these meta-analyses were published, there have been analyses examining either the validity or the magnitude of this relationship, and others comparing osteoporosis therapies that increase BMD between bisphosphonates but without fracture data. Some of the differences among published conclusions may be related to real differences between or among agents, differences in compliance, or differences in study design or study analysis. The uncertainty between the importance of the first fiddle versus the second fiddle, understandably, has led the U.S. Food and Drug Administration (FDA) to still require evidence for fracture risk reduction over 3 years as compared to placebo for registration of treatments for PMO—a prohibitively expensive, and perhaps unethical, place for patients, clinical investigators, and industry research and development to be put at this time. The search for the replacement of the first fiddle and an accurate surrogate marker of bone strength has been intense, with promising data suggesting that perhaps a more perfect surrogate marker can be found. Promising data suggest that a more trustworthy surrogate marker of bone strength, such as finite element analysis (FEA), could substitute for fracture endpoints. In addition, the FDA requires, in all registration trials, evidence of normal bone histomorphometry, because one of the reasons that higher doses of fluoride-induced increases in BMD may not be associated with risk reduction is the abnormal, ‘‘woven’’ bone and increased osteoid seen on bone histomorphometry. It seemsmore evident that if BMDmeasurement performed by dual-energy X-ray absorptiometry (DXA) declines beyond the in vivo least significant change (LSC) as compared to patients whose BMD remains stable or increases, a fracture benefit is minimized. Thus, in clinical practice, serial BMD measurements performed by DXA at 2-year intervals in the osteoporotic population on therapy remains the practitioners first fiddle. 29) Biochemical markers of bone turnover (BTM) that also reflect the pharmacological response(s) to osteoporosis therapies to reduce fracture risk, at least in groups of patients, are certainly in the orchestra and may take an increasingly important role in monitoring: perhaps, now, the third fiddle. For individual patient management, there must be improvements in the BTM assay standardization, harmonization, and reference population databases before they become a trusted third fiddle. In this issue of the JBMR, Jacques and colleagues report that, once again, there appears to be a robust relationship between the increase in total hip BMD as measured by DXA and the reduction in fracture risk with annual (5mg) IV zoledronic acid. The authors also found that the third fiddle retained its COMMENTARY JBMR