In the first study Neesha Rockwood and colleagues from the Chelsea and Westminster Hospital in west London did a retrospective case note review of patients attending the clinic between May 2006 and February 2010, singling out those who had had an abdominal X-ray or CT scan, a renal ultrasound scan, or an intravenous urogram (in which a contrast medium is injected and then x-rays are taken of it being excreted via the urinary system). These four procedures indicate that kidney stones are being investigated as a possible diagnosis.
She only included in this study patients who underwent one of the above investigations and were taking either the non-nucleoside drug (NNRTI) efavirenz (Sustiva, and in Atripla) or one of the protease inhibitors (PIs) atazanavir, lopinavir or darunavir (Reyataz, Kaletra, Prezista).
There have been case reports of kidney stones in patients taking atazanavir and analysis of these stones showed that they consisted of drug that has dissolved out of urine: atazanavir is less soluble in alkaline fluids, which is why it must also not be taken with antacids.
A sub-analysis excluded people with previous exposure to the PI indinavir (Crixivan), which is now hardly used because of its tendency to cause kidney stones. In all over 6,000 patients’ notes were reviewed. Of these 1206 people had taken atazanavir, 2803 efavirenz, 828 lopinavir and 818 darunavir, and 206 had previously taken indinavir.
Two per cent of patients taking atazanavir developed kidney stones, at an annual rate of 0.73% of patients per year (an annual event rate of one case per 137 patients). In contrast 0.54% of patients on the other three drugs developed kidney stones, an event rate of 0.19% or one event per 526 patients.
When patients with previous indinavir exposure were excluded, this brought the percentage of patients with kidney stones on atazanavir to 1.5%, with an annual event rate of 0.46% (one in 217 patients a year). The annual event rate in patients on the other three drugs who had not taken indinavir was 0.12% (one in 833 patients a year).
This meant that patients on atazanavir were 3.85 times as likely to get kidney stones as patients on the other drugs, and although excluding previous indinavir users brought the rates down, the relative difference remained the same.
Patients on darunavir were somewhat more likely to get kidney stones than patients on lopinavir or efavirenz (annual event rates 0.45%, 0.19% and 0.15% respectively).
Patients who had kidney stones were ten times more likely than others to have histories of chronic renal impairment and their levels of the liver waste product bilirubin (which causes jaundice, another side-effect associated with atazanavir) were on average twice as high.
Dr Rockwood said her study probably underestimated the prevalence of kidney stones in patients as some stones were transparent to X-rays and cases in patients who had transferred from other hospitals could have been missed. Atazanavir could have consequences after it was stopped: in one case, a kidney stone containing atazanavir was found in a patient who had stopped the drug 21 months previously.