Bone Disease and Pathologic Fractures in a Patient With Tenofovir-Induced Fanconi Syndrome

The NRTI tenofovir is currently recommended as part of an antiretroviral regimen for initial therapy and is widely used. Unlike the related medications cidofo­vir and adefovir, tenofovir was initially felt to be less nephrotoxic when giv­en at therapeutic doses.^1,2 However, it has recently become clear that there is a relationship between tenofovir and Fanconi syndrome (diffuse proximal tubule dysfunction), although this is a relatively rare side effect of this antiretroviral agent.^3-20 Tenofovir appears to be toxic to the renal tu­bules and can cause a variety of clinical effects in addition to Fanconi syn­drome, including disorders of urinary concentrating ability^12,14,21 and acute tubular necrosis.^{4,8,9,12,14,19,22-26}

We present a patient with a history of bone disease and long-term tenofovir therapy who suffered from Fanconi syndrome. We suggest that her tenofovir-induced Fanconi syndrome may have worsened her preexisting bone disease and contrib­uted to her subsequent pathologic fractures.

CASE SUMMARY
A 45-year-old HIV-infected woman presented to the emergency department with lower extremity swelling and a modestly elevated serum cre­at­i­­nine level (1.5 mg/dL). A work­up for possible nephrotic syndrome was initiated but did not reveal ne­phrot­ic-range proteinuria; however, the findings suggested Fanconi syndrome.

The patient, in whom HIV/AIDS had been diagnosed in early 1997, had been treated since mid-1997 with an antiretroviral regimen consisting of the protease inhibitor (PI) nelfin­avir plus lamivudine and zidovudine. In late 2002, the patient’s CD4⁺ cell count remained stable on this regimen at 158/µL, but her viral load was increasing.

HIV genotyping demonstrated viral resistance to all 3 of her current antiretroviral medications, possible resistance to didanosine, and sensitivity to tenofovir, lopinavir, and ritonavir. Between late 2002 and early 2003, the patient’s treatment was switched to a boosted PI regimen of lopinavir/ritonavir 200 mg/50 mg twice daily, tenofovir 300 mg once daily, and didanosine 250 mg once daily. Her CD4⁺ count while she was on this new regimen stabilized at 187/µL, and her HIV RNA level became undetectable.

Before the initiation of tenofovir therapy, her bone disease had not been diagnosed; however, after sustaining a fall in 2002, a tibia/fibula x-ray film demonstrated an incidental finding of bone loss consistent with osteoporosis, malnutrition, or chronic disease. There was no fracture, and the patient was asymptom­atic at the time. Of note, the patient (now receiving methadone maintenance ther­apy) had multiple risk factors for bone disease, including a history of injection drug use, oligomenorrhea resulting from hypogonado­trop­ic hy­pogonadism, and a low body weight (43 kg [95 lb]).

Two years after beginning her tenofovir-containing regimen, the patient complained of right hip pain and difficulty in walking after lifting a heavy object. Diagnostic imaging demonstrated an atraumatic pelvic fracture. An orthopedic consultation was obtained, but no surgery was recommended at that time because callous formation had already occurred. The patient began calcium/vitamin D supplementation.

Results of the patient’s urinalysis were normal, and her 25-hy­droxy­vitamin D level was low to normal at 23.4 ng/mL (normal, 20 to 100). However, no further testing for possible urinary phosphorus wasting was performed, and levels of active 1,25-hydroxyvitamin D metabolism were not measured.

Over the next several months, the patient complained of more generalized bone pain, including in her left shoulder, right ankle, and chest. She was found to have sustained multiple atraumatic rib fractures, and she began NSAID treatment for pain but discontinued treatment after a few weeks. In April 2005, 3 months after her pelvic fracture (approximately 27 months after beginning tenofovir treatment), the patient was hospitalized after acute tubular necrosis developed following an epi­sode of cocaine intoxication.

The patient’s serum creatinine level, which had always been normal, rose to 6.2 mg/dL (normal, 0.4 to 1.3). After several days of intravenous hydration, the patient was discharged with a slightly elevated creatinine level (1.3 mg/dL) and without interrupting her antiretroviral therapy.

During routine monthly outpatient follow-up, her serum creatinine levels remained mildly but persis­tently elevated, ranging from 1.5 to 2.0 mg/dL over the next 8 months. By November 2005, a nonanion gap metabolic acidosis also had developed, with bicarbonate levels of 14 to 15 mEq/L (normal, 20 to 30). No interval urinalysis was performed. The patient continued to complain of bone pain, and her alkaline phosphatase levels increased to 516 U/L (normal, 40 to 130) in the context of normal liver function test results. At this time, the patient also began complaining of lower extremity swelling and pain and was referred to a ne­phrologist for further workup for an elevated creatinine lev­­­el and possible nephrotic syndrome.

Before her scheduled appointment with a nephrologist, the patient presented to the emergency department in December 2005 with lower extremity swelling and a modestly elevated creatinine level (1.5 mg/dL) and was hospitalized-approximately 8 months after her episode of acute tubular necrosis. At this time, there was concern about possible pro­teinuric renal disease, such as HIV nephropathy. Urinalysis revealed glucosuria (despite a normal serum glucose level) and proteinuria (protein level of 70 mg/dL; normal, less than 10 mg/dL).

Further evaluation with a 24-hour urine sample demonstrated pro­teinuria (1.4 g), glucosuria (3.5 g), and phosphaturia (510 mg) despite a serum phosphorus level of 2.6 mg/dL. Urine protein electrophoresis demonstrated a tubular proteinuria pattern, and measurement of urinary ₂-microglobulin was 8500 µg/L (normal, 0 to 268).

As a result, Fanconi syndrome-likely related to treatment with tenofovir-was diagnosed, and her antiretroviral therapy was stopped. Before this hospitalization, she had been on her tenofovir-containing regimen for approximately 3 years (Figure 1). At discharge, raloxifene was prescribed for her bone disease.

One month after hospitalization, she was readmitted with severe right hip pain of 2 days’ duration after lifting a heavy object. Evaluation revealed a new right femoral neck fracture, and the patient underwent surgery for right hip hemiarthroplasty.

On routine follow-up in March 2006, 3 months after discontinuation of tenofovir, her serum creatinine level had normalized and her urinary abnormalities resolved with no further glucosuria and improved proteinuria (less than 500 mg/d) by spot protein-to-creatinine ratio. Her serum phosphorus level had normalized as well.

DISCUSSION
This case provides another example of Fanconi syndrome associated with tenofovir therapy. This patient’s glucosuria, phosphaturia, acidosis, and hypophosphatemia are all characteristic of proximal tubular cell injury. Results of urine protein electrophoresis also show a characteristic tubular injury pattern (Figure 2). Of equal concern is the substantial osteoporosis with pathologic fractures that she suffered.

The patient described here had multiple risk factors for the initial development of osteoporosis. The incidences of osteopenia and osteoporosis are increased in HIV-positive patients.^27-35 The cause of the reduced bone mineral density is not known, but both HIV infection itself^36-42 and antiretroviral therapy,^43-45 as well as impaired vitamin D metabolism,^32,46 low body weight, oligomenorrhea,^32,47-48 and opiate use,^49-51 are risk factors for early osteoporosis.

Our patient had all of these risk factors, which explains the development of significant bone disease at age 45, before the onset of tenofovir-associated Fanconi syndrome. It is impossible to link definitively the patient’s tenofovir exposure to her pathologic fractures because she had earlier evidence of bone loss. Nevertheless, the negative phosphorus balance induced by her Fanconi syndrome and other factors related to tenofovir exposure, such as metabolic acidosis and impaired vitamin D metabolism, may have contrib­uted to accelerated bone loss and development of pathologic fractures.

In reviewing this case, the point at which we should have stopped the patient’s tenofovir therapy is when her creatinine level did not return to baseline after her episode of acute renal failure. With her low body weight, the patient’s creatinine levels of 1.5 to 2.0 mg/dL did not appear abnormal; however, her glo­mer­ular filtration rate (GFR) was approximately 38 to 40 mL/min/1.73m² using the Modification of Diet in Renal Disease (MDRD) equation (24 to 31 mL/min using the Cockcroft-Gault formula).

Given tenofovir’s renal excretion through both filtration and active secretion, the dosage of this agent should be altered for creatinine clearances of less than 50 mL/min.^3,52 Concurrent use with the lopinavir/ritonavir coformulation has also been documented to increase tenofovir levels.^4,12,53-58 Continued administration of tenofovir with reduced renal clearance of the drug secondary to an impaired GFR with concurrent use of lopinavir/ritonavir may have predisposed our patient to the drug’s dose-dependent toxicities and may have been a factor in the development of Fanconi syndrome.

This patient’s tenofovir-associated Fanconi syndrome may have contributed to her bone disease in several ways. Severe bone pain and spontaneous pathologic fractures are char­acteristics of adult-onset Fanconi syndrome.^1,11,59,60 The proposed mechanisms for bone disease in patients with adult-onset Fanconi syndrome include negative phospho­-rus balance from renal phosphorus wast­ing, chronic acidosis, and impaired vitamin D metabolism.⁵⁹

Osteomalacia results when prox­imal tubular cells are dysfunctional in both reabsorptive capacity and metabolism of vitamin D, specifically the 1-hydroxylation of 25-hydroxyvitamin D into its most active form, calcitriol.^61-63 Hypophospha­te­mic osteomalacia presenting with bone pain and myalgias has also been observed in 4 other HIV-positive patients with tenofovir-induced Fanconi syndrome.^5,13,64

Tenofovir therapy in the absence of renal insufficiency has been associated with high levels of active 1,25-hydroxyvitamin D.³ There do not appear to be studies looking into the relationship between vitamin D metabolism and tenofovir-induced Fanconi syndrome, nor is there much literature on the relationship between tenofovir exposure and daily phosphorus balance, with or without re­nal impairment.

It is possible that our patient’s bone disease before her frank Fanconi syndrome may reflect a milder degree of medication toxicity not detected by our hospital’s usual screening mechanisms, such as urinalysis and serum creatinine measurements. This milder toxicity would have contributed to her earlier pelvic fracture and bone disease. At the time of her first pelvic fracture, the patient had a low to normal 25-hydroxyvitamin D level. However, this normal level would not rule out a low level of calcitriol if vitamin D activation in the proximal tubules was impaired. It is also conceivable that the patient may have been in chronically negative phosphorus balance if re-nal reabsorption of phosphorus was impaired.

Given the many possible effects of tenofovir on the renal tubules with regard to acid-base balance, phosphorus handling, and vitamin D metabolism, we hypothesize that te­no­fovir may cause subclinical alterations in daily phosphorus balance that accelerate bone loss or adversely affect vitamin D activation, even in patients who do not have evidence of Fanconi syndrome. HIV disease itself is associated with alterations in the activation of vitamin D,^{32,41-43,46,65} and further disruption in the production of calcitriol by tenofovir therapy could worsen bone disease. Unfortunately, in our patient, a calcitriol level was not obtained, nor do we have information about 24-hour phosphorus excretion before the development of full-blown Fanconi syndrome.

As has been demonstrated by earlier animal studies and case reports, tenofovir appears to have a dose-related toxicity regarding renal tubular cells.^3,53,60,66 Our patient’s time course also suggests a dose- dependent toxicity, with osteomalacia occurring during normal renal function, but Fanconi syndrome developing after her episode of acute renal failure and subsequently reduced GFR. After her acute renal failure, the patient’s impaired creatinine clearance would cause an accumulation of tenofovir in the blood, predisposing her to development of frank Fanconi syndrome.^5,53,60

Tenofovir therapy has been associated with increased bone turn­over and decreased bone mineral density.^3,67 In animal studies, high-dose tenofovir exposure has been associated with an osteomalacia-like condition in the setting of normal re­nal function, resulting in spontane­ous fractures and bone defor­mi­ties.^68,69 Tenofovir therapy has also been associated with the devel­op­ment of mild hypophosphatemia in patients who have normal renal function.^9,70-72

A study by Gallant and coworkers⁶⁷ suggested that tenofovir itself may have a statistically significant association with increased bone loss compared with other antiretroviral agents; however, the bone loss seen in their study was nonprogressive, of a small magnitude, and with unclear clinical significance. There has also been an association between tenofovir treatment and decreased bone mineral density in a study of children, although the association did not reach statistical significance.⁷³

Tenofovir-induced renal damage appears to be quite rare. Although small retrospective studies have observed evidence of tenofovir-induced tubular damage (Fanconi syndrome or acute tubular necrosis) in 2% to 4% of patients studied,^9,74 a large randomized controlled trial of 600 patients (of which, 299 received tenofovir) failed to demonstrate a significant trend toward nephrotox­icity related to the use of tenofovir.⁷⁰ Because of tenofovir’s variety of effects on tubular cells, some of which are not detected by the most commonly ordered laboratory assessments, a more uniform screening protocol to identify patients at risk for tenofovir-related nephrotoxicity should be adopted (Figure 3). Tenofovir does appear to be safer than its cousins, adefovir and cidofovir, and can still be used safely, provided precautions are taken and appropriate screening is performed.

Baseline GFR estimates should be obtained for patients in whom tenofovir therapy is being considered and for those who are already receiv­ing therapy. This can be done with either the MDRD equation or the Cock­croft-Gault formula. Patients with GFR estimates above 50 mL/min/1.73m² body surface area are probably at reduced risk for toxicity.³ Patients with lower GFR estimates clearly have impaired renal function and need to be monitored more closely.

The HIV Medicine Association of the Infectious Diseases Society of Amer­­ica and others recommend adjusting tenofovir dosing based on cre­atinine clearance, with once-daily 300-mg dosing lengthened to every 48 hours for creatinine clearances be­tween 30 and 49 mL/min and to ev­ery 72 hours for 10 to 29 mL/min.^3,75-77

Measuring urinary ₂-micro­globulin levels may identify patients with evidence of tubular toxicity, although this may not necessarily predict outcomes (until more research in this area is completed). Kinai and Hanabusa64 found that increased urinary ₂-microglobulin levels after initiating tenofovir therapy were cor­related with a decrease in the tubular reabsorption of phosphorus.

To identify patients at risk for Fanconi syndrome and other proximal tubular effects, it may be helpful to obtain a urinalysis to screen for glucosuria. Monitoring for reductions in serum bicarbonate and phos­pho­rus levels might also be helpful; however, serum phosphorus levels may lag behind the onset of osteomalacia. For detecting the phos­pho­rus wasting in Fanconi syndrome, 24-hour urine studies that monitor for phosphaturia and the fractional excretion of urinary phosphorus can be useful.

In the study by Kinai and Hana­busa,⁶⁴ tubular reabsorption of phosphorus lower than 90% was noted in the tenofovir-treated group but not in the control group. This finding suggests that this level would be an appropriate set point for screening, although the specificity of this test may be low given that urinary phosphorus levels are influenced by a multitude of factors.

Serum creatinine levels alone may not be a reliable indicator of toxicity. As earlier studies have shown, there may be a modest reduction in GFR estimates in patients treated with tenofovir therapy^6,9,70; however, tubular toxic­ity would need to be quite severe to result in a substantial increase in serum creatinine levels, and many of the effects noted above can occur in patients with relatively preserved re­nal function.

The case reported here illustrates the need to monitor for bone disease. As mentioned above, persons with HIV/AIDS have many potential causes of bone disease, including low body weight, oligomenorrhea, history of atraumatic fractures, bone pain, opiate use, kidney disease, and long-term corticosteroid treatment; those at risk should be monitored closely when receiving a tenofovir-containing regimen.

For identification of bone disease, baseline and follow-up bone densitometry would be appropriate. Measurement of 1,25-hydroxyvitamin D (calcitriol) levels may also be useful because 1,25-hydroxyvitamin D directly reflects synthesis of active vitamin D by the proximal tubular cells. Monitoring serum alkaline phos­phatase levels may also be helpful in screening for bone disease.^5,64

Treatment of bone disease in HIV-positive patients with tenofovir-induced Fanconi syndrome can involve several approaches. Demonstration of urinary phosphorus and glucose wasting or highly elevated levels of urinary ₂-microglobulin suggest proximal tubular injury and should prompt discontinuation of tenofovir. Abnormalities in markers of bone turnover or bone density should prompt closer follow-up of bone disease.

It is unclear at this time whether such therapies as calcium and vitamin D supplementation, selective estrogen receptor modulators, and bisphosphonates are helpful in this setting or whether discontinuation of tenofovir is warranted. Supplementation with calcium and vitamin D has been shown to be beneficial in patients with Fanconi syndrome despite impaired re­nal vitamin D activation^13,59; however, this may be less effective in HIV-infected patients who also have impaired activation of vitamin D because of their virus or antiretroviral drug regimen.

Bisphosphonates have been shown to be effective in HIV-positive patients in the treatment of osteo­penia and osteoporosis, especially in im­proving bone mineral density at the hip; however, bisphosphonates should not be used by patients with a creatinine clearance of less than 30 mL/min, to avoid the risk of drug accumulation.^31,78,79 Caution should also be used with selective estrogen receptor modulators, such as raloxifene, that can inhibit the cytochrome P-450 metabolism of many antiretroviral agents.³⁵

The present case provides an example of the risk of tenofovir-induced Fanconi syndrome. In our patient, we hypothesize that this com­plication worsened her preexisting bone disease. This case underscores the importance of not only carefully monitoring renal function in a patient treated with a tenofovir-containing regimen but also monitoring for bone disease.

Dr Cu-Uvin reports having served as a regional consultant for Boehringer Ingelheim and Bristol-Myers Squibb; received research support from Bristol-Myers Squibb; and received honoraria from GlaxoSmithKline. No other potential conflict of interest relevant to this article was reported by the authors.

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