# TZDs and Bone: A Review of the Recent Clinical Evidence

Authors: Ann V. Schwartz
Journal: PPAR Research
 (2008)

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## Abstract

Over the past two years, evidence has emerged that the currently available thiazolidinediones (TZDs), rosiglitazone, and pioglitazone have negative skeletal consequences, at least in women, which are clinically important.  Increased fracture risk in women, but not men, was reported for both TZDs, based on analyses of adverse event reports from clinical trials.  In short-term clinical trials in women, both TZDs caused more rapid bone loss.  In these trials, changes in bone turnover markers suggest a pattern of reduced bone formation without a change in resorption.  Although limited, these results support the hypothesis based on rodent and in vitro models that reduced bone formation resulting from activation of peroxisome proliferator-activated receptor-γ (PPARγ) is a central mechanism for TZDs_ effect on bone.  Research is needed to better understand the mechanisms of bone loss with TZDs, to identify factors that influence susceptibility to TZD-induced osteoporosis, and to test treatments for its prevention.


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## Body

## 1. Introduction

Recent
reports have substantially advanced our knowledge of the clinical effects of
TZDs on skeletal health. In early 2006, research
into the skeletal effects in humans of rosiglitazone and pioglitazone, the currently
prescribed TZDs, was limited to observational studies [1]. Although a body of evidence had developed
from rodent and in vitro studies that these two TZDs cause bone loss, it was
not known if these compounds had a similar effect in humans. Since then, rosiglitazone and piogltiazone were
each linked to increased fracture risk among diabetic women, based on adverse
event reports in clinical trials. And,
in women, short-term clinical trials demonstrated substantial bone loss with
both TZDs. Pioglitazone and
rosiglitazone are widely used to treat diabetes, and better knowledge of their
skeletal effects is crucial to guide clinical decisions. At the same time, because TZDs are ligands of
PPARγ, a better understanding of their skeletal effects will help to clarify
the role of PPARγ in bone metabolism and potentially shed light on the
mechanisms of age-related bone loss. This
review considers the recent clinical evidence regarding TZDs and skeletal
health and discusses outstanding issues that warrant further research.


## 2. Rosiglitazone and Fracture Risk

Evidence
that RSG increases fracture risk emerged with the results of the ADOPT trial
published in 2006 [2]. A postproof note in the main report from the
trial indicated increased fracture risk in women, but not men, enrolled in the
trial. Since then, the fracture results
have been published separately and in more detail [3]. ADOPT was designed to assess time to
monotherapy failure for RSG compared to metformin and to a sulfonylurea,
glyburide. The trial had three arms,
corresponding to the three different treatments, and enrolled a total of 2511
men and 1840 women who were followed for a median of 4.0 years. The average age was 57 years. By
self-report, 77% of women were postmenopausal.
Participants were recently diagnosed with diabetes (<3 years), were
drug naïve for hypoglycemic medications, and had an average A1C of about
7.4%.Fractures,
identified through adverse event reports, were specifically reviewed after the
conclusion of the trial. Based on time
to first fracture, the investigators found an increased risk among women in the
RSG arm of 1.81 (95% CI: 1.17, 2.80) compared to metformin, and 2.13 (1.30,
3.51) compared to glyburide. The risk
for men was not increased compared with either metformin (RH 1.18; 95% CI:
0.72, 1.96) or glyburide (RH 1.08: 95% CI: 0.65, 1.79).In women,
risk was increased for both upper and lower limb fractures. Rate ratios calculated from
fracture rates
reported for ADOPT showed the largest increases in relative risk for foot(RR=3.3),
hand (RR=2.6), and proximal humerus (RR>8) fractures (see Table 1). There was no increased risk identified for
clinical spine or hip fractures, but the numbers of these fractures, 3 clinical
spine and 4 hip fractures among all women, were too small to draw firm
conclusions. The small number of hip
and spine fractures in the ADOPT population (average age 57 years) is not
surprising since the rate of these fractures tends to be relatively low until
after age 65.Table 1
Fracture rates comparing rosiglitazone with metformin or glyburide in
ADOPT study. Table adapted from a
Letter to Health Care Providers issued by GSK [4].
RosiglitazoneMetformin or glyburideRelative rate (95% CI)WomenTotal followup (P-Y)2187.203578.80Fracture siteNRate/100PYNRate/100PYRR(95% CI)Lower limb*361.65260.732.27(1.33, 3.91)Hip20.0920.061.64(0.12, 22.57)Foot221.01110.313.27(1.52, 7.47)Upper limb†221.01190.531.89(0.98, 3.70)Hand80.3750.142.62(0.76, 10.17)Humerus50.2300.00‡(1.50,‡)Spine10.0520.060.82(0.01, 15.72)Other50.2380.221.02(0.26, 3.55)All fractures642.93551.541.90(1.31, 2.78)MenTotal followup (P-Y)2766.705570.40NRate/100PYNRate/100PYRR(95% CI)Total participants with any fracture321.16571.021.13(0.71, 1.77)* Hip, foot, ankle, femur, fibula, lower
limb (general), patella, tibia.
† Hand, humerus, clavicle, forearm, radius,
upper limb (general), wrist.
‡ Cannot estimate. No events in the comparison group.
Reprinted with permission from [5]For
women, an examination of the survival curves from the ADOPT trial (see Figure1)
suggests that the increased risk of fracture with RSG is evident after about
one year of treatment. In separate
trials, discussed below, bone loss could be identified among women treated with
RSG after only a few months of treatment. However, the ADOPT results suggest that bone
loss with RSG does not make a noticeable difference in fracture risk until after
about 12 months of treatment.Figure 1
Kaplan-Meier estimates
of the cumulative incidence of fractures at five years in women enrolled in
ADOPT [3]. Bars represent 95% confidence intervals.Self-reported
menopausal status and baseline use of estrogen-containing hormones were available
for women enrolled in ADOPT. As
expected, premenopausal women had a lower rate of fracture than postmenopausal
women, but both groups had an approximate doubling of fracture risk with RSG
treatment. Menopausal status did not
appear to substantially modify the effects of RSG on fracture. About 20% of women reported use of an
estrogen-containing hormone at baseline.
The effect of RSG on fracture risk did not appear to differ between
those who did or did not report estrogen use.It is
possible, though not established, that poor glycemic control increases fracture
risk [6]. However, this would not explain the ADOPT
results as those in the RSG arm maintained glycemic control on monotherapy
longer than those in the metformin or glyburide arms.


## 3. Pioglitazone and Fracture Risk

With
the published report of increased fracture risk in the RSG arm of ADOPT, Takeda Pharmaceuticals, IL, USA  the manufacturer of
pioglitazone, reviewed their clinical trial databases and, in a letter to
health care providers in 2007, reported an increased fracture risk with
pioglitazone treatment in women, but not men [7]. The databases included 24 000 years of
followup for over 8100 patients treated with pioglitazone and over 7400
patients in the comparison group. In
these trials, the maximum duration of pioglitazone use was only 3.5 years. The magnitude of the increased risk reported
for all clinical fractures was similar to the ADOPT results with a fracture
rate of 1.9 per 100 person years in those using pioglitazone compared with a
rate of 1.1 per 100 person years in those using placebo or an active comparator
drug. The relative risk for men was not
reported but was stated to be not statistically significant. Data on specific fracture sites was not
provided although the letter stated that most of the fractures occurred in the
distal upper limb or distal lower limb.


## 4.TZDs and Bone Loss

In 2007,
Grey et al. reported the results of a 14-week randomized clinical trial
comparing RSG (8 mg/day) with placebo in 50 postmenopausal women, average age 67
years, who did not have diabetes or osteoporosis [8]. The trial found modest reductions in two
markers of bone formation. Procollagen
type-I N-terminal propeptide was reduced by 13%(P=.004) and
osteocalcin by 10%(P=.04) in the RSG arm compared with placebo. In contrast, the bone resorption marker, serum
β-C-terminal telopeptide (S-CTX) of type I collagen, was stable in the RSG arm and
did not differ significantly from placebo (P=.9). Substantial bone loss was reported at the
total hip with RSG treatment. Women in
the RSG group lost bone density (BMD) more rapidly at the total hip (−1.9% RSG
versus −0.2% placebo, P=.003). For the total spine, bone loss was more rapid in the RSG arm but the
difference was not statistically significant (−1.2% RSG versus −0.2% placebo, P=.13).In a
randomized, controlled, but unblinded trial, a lower dose of RSG (4 mg/day) for
12 weeks was compared with diet treatment alone in obese postmenopausal women
with newly diagnosed diabetes [9]. Bone-specific alkaline phosphatase, a bone
formation marker, was decreased in the RSG arm (−21.5%) compared with diet only
(−4.1%) (P<.05). Osteocalcin
was decreased similarly in both arms (RSG −20%; diet only −17.6%) while urine
deoxypyridinoline (DPD), a resorption marker, was not increased in the RSG arm
(3%) compared with the diet only arm (17%).The
short-term effects of pioglitazone (30 mg/day) on bone density and markers have
been tested in a 16-week randomized placebo-controlled trial among 30
premenopausal women with polycystic ovary syndrome (PCOS) [10]. BMD was reduced compared with placebo at the
lumbar spine (−1.14% versus 0.00%), total hip (−0.18% versus 1.35%), and
femoral neck (−1.45% versus 0.87%) (all P<.05). The magnitude of loss in the PIO group at the
spine and femoral neck is similar to BMD losses reported with RSG use over 14
weeks in postmenopausal women [8]. Alkaline phosphatase, a marker of bone
formation, was decreased in the PIO group compared to placebo but osteocalcin
was not. Changes in the marker of bone
resorption, S-CTX, were also not significantly different across treatment
groups. The treated group experienced a significant
decrease in fasting insulin compared to placebo. Since insulin may be anabolic for bone, this
may have contributed to the bone loss observed with PIO although the authors reported
that the changes in BMD and the changes in insulin were not significantly
correlated. Estradiol and testosterone
levels were not significantly altered in the PIO group.Two
observational studies have reported results for TZDs and changes in BMD or
markers. The first clinical study to
report increased bone loss with TZD use, combining troglitazone, rosiglitazone,
and pioglitazone, was based on the Health, Aging, and Body Composition
longitudinal observational study of older adults [11]. The cohort included 666 diabetic participants
with an average age of 73 years. Of
these, 69 participants reported any TZD use during four years of followup. Increased bone loss was found in diabetic
women but not men. After controlling for
potential confounders, the additional bone loss attributed to TZD use in women was
−1.23% (95% CI: −2.06%, −0.40%) per year at the lumbar spine, −0.61% (−1.02%,
−0.21%) per year for whole body, and −0.49% (−1.04%, 0.07%) for total hip. These estimates of increased bone loss are
substantially lower than those reported by Grey et al. [8]
for the trial of RSG use and by Glintborg et al. [10]
for the trial of PIO use. The additional
bone loss of 1.5–1.7% at the total hip over 14–16 weeks in these
two trials, if sustained, would result in additional bone loss of 5-6% annually. While the observational study by Schwartz et
al. may have underestimated the degree of bone loss associated with TZD use, it
seems unlikely that bone loss of 6% per year is occurring with TZD use. Instead, there may be an initial period of more
rapid bone loss, followed by continued loss at a lower rate, similar to the
effect of glucocorticoids [12].Although
Schwartz et al. reported no increased bone loss with TZD use in diabetic men,
Yaturu et al., in an observational study of 160 older diabetic men (average age
68 years), did report that RSG use(N=32) was associated with increased bone
loss of −1.05% per year at the total hip, −1.02% at the femoral neck, and
−1.24% at the spine (all P<.03) [13]. However, the study did not have sufficient
power to control for potential confounders such as A1C level, use of other
medications, or diabetic complications.


### 4.1. Rodent and In Vitro Models

Results
of rodent and in vitro models provided the first evidence that RSG and PIO
cause bone loss. RSG has been more extensively
studied in these models but both compounds are associated with bone loss in
rodents [14, 15]. These findings have been reviewed previously
[16, 17]
and will not be discussed in depth here.
However, a few points are worth noting as particularly relevant to
future research in humans. In general,
these models indicate a negative effect on osteoblast differentiation and
activity with a decrease in bone formation. However, in a few reports, TZDs were
associated with increased resorption.
Notably, this occurred in ovariectomized rats [18]
and in aged mice [19]. Sottile et al. reported that ovariectomized
rats experienced bone loss with RSG, but intact female rats did not, and that
the bone loss was characterized by increased resorption [18]. This suggests an interaction between RSG and
estrogen levels that needs to be assessed in human studies. The results from Lazarenko et al. comparing
the effects of RSG in young, adult, and aged mice suggest that the mechanism of
action may be different in the aged mice [19]. In young and adult mice, bone loss with RSG
treatment was driven by reduced formation while in older mice RSG treatment
resulted in increased resorption. These
results need to be explored in human studies as they would suggest different
approaches to treatment for the prevention of TZD-induced osteoporosis.


## 4.1. Rodent and In Vitro Models

Results
of rodent and in vitro models provided the first evidence that RSG and PIO
cause bone loss. RSG has been more extensively
studied in these models but both compounds are associated with bone loss in
rodents [14, 15]. These findings have been reviewed previously
[16, 17]
and will not be discussed in depth here.
However, a few points are worth noting as particularly relevant to
future research in humans. In general,
these models indicate a negative effect on osteoblast differentiation and
activity with a decrease in bone formation. However, in a few reports, TZDs were
associated with increased resorption.
Notably, this occurred in ovariectomized rats [18]
and in aged mice [19]. Sottile et al. reported that ovariectomized
rats experienced bone loss with RSG, but intact female rats did not, and that
the bone loss was characterized by increased resorption [18]. This suggests an interaction between RSG and
estrogen levels that needs to be assessed in human studies. The results from Lazarenko et al. comparing
the effects of RSG in young, adult, and aged mice suggest that the mechanism of
action may be different in the aged mice [19]. In young and adult mice, bone loss with RSG
treatment was driven by reduced formation while in older mice RSG treatment
resulted in increased resorption. These
results need to be explored in human studies as they would suggest different
approaches to treatment for the prevention of TZD-induced osteoporosis.


## 5. Future Directions for Clinical Research

Substantial
evidence has now emerged that RSG and PIO have clinically important negative
skeletal effects. Increased fracture
risk in women, but not men, has been reported for both RSG and PIO. Although this increased fracture risk was
identified in the context of clinical trials, the fractures were identified
through adverse event reports and were not a planned outcome of the
trials. It is possible for adverse event
results in a clinical trial to give a signal that is statistically significant
due to chance rather than to an actual effect of the intervention. However, the fracture effect is consistent
with two clinical trials demonstrating bone loss with RSG and PIO. And, the increased fracture risk and bone
loss are consistent with the results of rodent and in vitro models. The combination of these studies provides a
compelling argument that, in women, the two currently prescribed TZDs cause
higher fracture risk due to bone loss.Given
this growing evidence of increased fracture risk and bone loss with TZD use, further
exploration of the skeletal effects of TZDs is crucial to inform efforts to
prevent TZD-induced osteoporosis and, more generally, to delineate the role of
PPARγ in bone metabolism. Some of the
key questions for clinical research are identified and discussed below.


### 5.1. What Groups are at Higher Risk?

To
inform clinical decisions and to better understand the mechanism of TZDs effects
on the skeleton, it is important to ascertain if there are groups that are
particularly vulnerable, or groups that are not susceptible, to increased
fracture risk with TZD use. So far, the
negative skeletal effects seem to be more important for women than for men, but
results are not conclusive. Among
women, menopausal status does not appear to modify the effect of RSG on the
skeleton. The ADOPT results indicate
that increased fracture risk extends to those who are premenopausal as well as
postmenopausal. Both premenopausal [10]
and postmenopausal [8]
women have been shown to lose bone with TZD treatment.A
possible explanation for the lack of effect on the skeleton in men is the
higher estrogen levels found in older men compared with older women. In a rat model, ovariectomized, but not
intact, females had bone loss with RSG treatment, suggesting a protective
effect from higher estrogen levels [18]. However, clinical results to date indicate
that TZDs cause increased bone loss and fracture risk in pre- as well as
postmenopausal women. Further research with
measurements of endogenous estrogen levels could clarify whether there is an
interaction between estrogen levels and TZD use.


### 5.2. What Happens to Bone Density and Turnover after 3-4 Months of
Treatment?

The randomized
trials with RSG and PIO have reported on treatment for 14–16 weeks. In both trials, the additional bone loss in
the treated group was substantial, equivalent to a loss of 5-6% over a year,
but it seems unlikely that this rate of loss is being sustained over longer
treatment periods. Observational studies
suggest increased loss of about 0.5–1% each
year. Steroid treatment appears to cause
initial rapid bone loss followed by continued loss but at a lower rate; the TZDs may present a similar pattern [12]. However, trials of longer duration are needed
to assess the degree of loss over several years.


### 5.3. Effect on Resorption as Well as Formation?

One
of the key questions regarding the mechanism of action of the TZDs is whether
bone resorption and formation, or only one, are affected. The clinical evidence to date, based on bone
turnover markers, points to a reduction in bone formation without a change in
bone resorption. However, these results
are based on only three studies that included bone marker results [8–10]. Rodent models have
generally shown reduced bone formation but, in aged mice and in ovariectomized
rats, bone resorption is increased.
Whether bone resorption is similarly increased with older age or with
very low endogenous estrogen levels in human studies has not been fully
explored.


### 5.4. Do Effects on Cortical and Trabecular Bone Differ?

The
increased fracture risk observed in the bones of the extremities, that have a
relatively high proportion of cortical bone, suggests a negative impact on
cortical bone. This pattern is distinct
from glucocorticoids which have a particularly strong effect on trabecular bone
and the risk of vertebral fracture [12]. Studies using imaging techniques that can separate
these two compartments, such as high resolution computed tomography, could
clarify whether the effects of TZDs differ for cortical and trabecular bones.


### 5.5. Marrow Adiposity

In
most reports from rodent models, increased marrow adiposity accompanies bone
loss with RSG treatment. Further
investigation of this phenomenon has suggested that activation of PPARγ with
RSG increases lineage allocation of stem cells towards adipocytes at the
expense of osteoblasts in the marrow. To date, human studies have not measured bone marrow adiposity. Knowledge of the effect of TZDs on bone
marrow fat would increase our understanding of the mechanisms underlying bone
loss and fracture risk in humans with TZD use.
In addition, an increase in bone marrow fat may cause an artificial
decrease in BMD measured by DXA [20]. If marrow fat is increased, the degree
of bone loss with TZD use may be overestimated by DXA measurements.


### 5.6. Effective Treatment for TZD-Induced Osteoporosis

There
are no studies to date on treatments that might prevent TZD-induced bone
loss. Although the bisphosphonates
mainly target bone resorption, the general reduction in bone turnover may be
efficacious in preventing bone loss with TZD treatment. The bisphosphonates are successfully used for
prevention of osteoporosis with corticosteroid treatment, also characterized by
reduced bone formation [21]. However, TZDs have specific effects on bone,
and bisphosphonate use should be explicitly tested to determine efficacy in
this situation. Treatments
that increase bone formation, currently limited to parathyroid hormone (PTH)
and strontium ralenate, could theoretically prevent TZD-induced bone loss. PTH has been shown to prevent bone loss with
glucocorticoid therapy [22],
but neither treatment has been tested in relation to TZDs.


## 5.1. What Groups are at Higher Risk?

To
inform clinical decisions and to better understand the mechanism of TZDs effects
on the skeleton, it is important to ascertain if there are groups that are
particularly vulnerable, or groups that are not susceptible, to increased
fracture risk with TZD use. So far, the
negative skeletal effects seem to be more important for women than for men, but
results are not conclusive. Among
women, menopausal status does not appear to modify the effect of RSG on the
skeleton. The ADOPT results indicate
that increased fracture risk extends to those who are premenopausal as well as
postmenopausal. Both premenopausal [10]
and postmenopausal [8]
women have been shown to lose bone with TZD treatment.A
possible explanation for the lack of effect on the skeleton in men is the
higher estrogen levels found in older men compared with older women. In a rat model, ovariectomized, but not
intact, females had bone loss with RSG treatment, suggesting a protective
effect from higher estrogen levels [18]. However, clinical results to date indicate
that TZDs cause increased bone loss and fracture risk in pre- as well as
postmenopausal women. Further research with
measurements of endogenous estrogen levels could clarify whether there is an
interaction between estrogen levels and TZD use.


## 5.2. What Happens to Bone Density and Turnover after 3-4 Months of
Treatment?

The randomized
trials with RSG and PIO have reported on treatment for 14–16 weeks. In both trials, the additional bone loss in
the treated group was substantial, equivalent to a loss of 5-6% over a year,
but it seems unlikely that this rate of loss is being sustained over longer
treatment periods. Observational studies
suggest increased loss of about 0.5–1% each
year. Steroid treatment appears to cause
initial rapid bone loss followed by continued loss but at a lower rate; the TZDs may present a similar pattern [12]. However, trials of longer duration are needed
to assess the degree of loss over several years.


## 5.3. Effect on Resorption as Well as Formation?

One
of the key questions regarding the mechanism of action of the TZDs is whether
bone resorption and formation, or only one, are affected. The clinical evidence to date, based on bone
turnover markers, points to a reduction in bone formation without a change in
bone resorption. However, these results
are based on only three studies that included bone marker results [8–10]. Rodent models have
generally shown reduced bone formation but, in aged mice and in ovariectomized
rats, bone resorption is increased.
Whether bone resorption is similarly increased with older age or with
very low endogenous estrogen levels in human studies has not been fully
explored.


## 5.4. Do Effects on Cortical and Trabecular Bone Differ?

The
increased fracture risk observed in the bones of the extremities, that have a
relatively high proportion of cortical bone, suggests a negative impact on
cortical bone. This pattern is distinct
from glucocorticoids which have a particularly strong effect on trabecular bone
and the risk of vertebral fracture [12]. Studies using imaging techniques that can separate
these two compartments, such as high resolution computed tomography, could
clarify whether the effects of TZDs differ for cortical and trabecular bones.


## 5.5. Marrow Adiposity

In
most reports from rodent models, increased marrow adiposity accompanies bone
loss with RSG treatment. Further
investigation of this phenomenon has suggested that activation of PPARγ with
RSG increases lineage allocation of stem cells towards adipocytes at the
expense of osteoblasts in the marrow. To date, human studies have not measured bone marrow adiposity. Knowledge of the effect of TZDs on bone
marrow fat would increase our understanding of the mechanisms underlying bone
loss and fracture risk in humans with TZD use.
In addition, an increase in bone marrow fat may cause an artificial
decrease in BMD measured by DXA [20]. If marrow fat is increased, the degree
of bone loss with TZD use may be overestimated by DXA measurements.


## 5.6. Effective Treatment for TZD-Induced Osteoporosis

There
are no studies to date on treatments that might prevent TZD-induced bone
loss. Although the bisphosphonates
mainly target bone resorption, the general reduction in bone turnover may be
efficacious in preventing bone loss with TZD treatment. The bisphosphonates are successfully used for
prevention of osteoporosis with corticosteroid treatment, also characterized by
reduced bone formation [21]. However, TZDs have specific effects on bone,
and bisphosphonate use should be explicitly tested to determine efficacy in
this situation. Treatments
that increase bone formation, currently limited to parathyroid hormone (PTH)
and strontium ralenate, could theoretically prevent TZD-induced bone loss. PTH has been shown to prevent bone loss with
glucocorticoid therapy [22],
but neither treatment has been tested in relation to TZDs.


## 6. Conclusion

Research
over the past two years has provided new clinical evidence that the currently
prescribed TZDs increase fracture risk and bone loss, at least in women. Combined with the findings from rodent and in
vitro models, these clinical results suggest that activation of PPARγ can play
a role in bone loss. With the widespread
use of TZDs as a diabetes treatment, further research is needed to delineate
the groups that are most susceptible to TZD-induced osteoporosis, to determine
the rate of bone loss with TZD treatment beyond 16 weeks, to assess the effects
of TZDs on marrow adiposity, cortical and trabecular bones, and to identify
treatments to prevent TZD-induced fracture risk. Addressing these questions will advance our
ability to prevent TZD-induced osteoporosis and will provide a better
understanding of the role of PPARγ activation in bone metabolism.



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*Source: 297893-2008-09-08.xml*