Oxycodone to Oxymorphone Metabolism
Primary care physicians, as well as pain specialists, are increasingly ordering urine drug tests as part of the initial evaluation and follow-up of patients with chronic pain when opioid therapy is being used or is under consideration. Physicians should know that ordering a urine drug test (UDT) carries with it an obligation to understand the results and to act on them accordingly, instituting changes in treatment plan if indicated.
Interpreting UDT results can be confusing unless physicians understand the metabolism of opioids. For example, it is well recognized that codeine is a pro-drug, with its analgesic effect resulting from conversion of codeine to morphine by the cytochrome P450 2D6.1,2 Thus, patients on codeine frequently test positive for both codeine and morphine. When patients who lack the cytochrome P450 2D6 enzyme necessary to convert codeine to morphine are treated with codeine, their urine may show only codeine.2 On the other hand, a finding of codeine in the urine of a patient being treated with morphine implies that the patient was also obtaining codeine from another source. Similarly, hydrocodone is metabolized to hydromorphone, so that both may legitimately be found in the urine of a patient who is being prescribed hydrocodone (Vicodin, Lorcet, etc.)3,4 But again, the reverse is not true; a patient prescribed hydromorphone (Dilaudid) should not have hydrocodone in the urine. Recently, Cone et al5 reported that in some patients chronically treated with morphine, hydromorphone can appear in the urine as a result of a minor metabolic pathway.
An extended-release oxymorphone (OpanaER) and an immediate-release oxymorphone (Opana) have recently become available. The question then arises, what is the explanation for a finding of oxymorphone in the urine of a patient who is not being prescribed this drug? Oxycodone is metabolized in part by cytochrome P450 2D6 to oxymorphone, which represents less than 15% of the total administered dose.6 However, oxymorphone has a significantly longer half life (7-9 hours)7 than does oxycodone, whose mean elimination half-life following a single, oral dose is 3.51 ± 1.43 hours.8 It is therefore plausible that in oxycodone-using patients, serum and urine levels of oxymorphone may be significantly more than 15% those of oxycodone. Large numbers of patients consume oxycodone, either as the extended-release form (OxyContin) or as immediate-release Percocet, Percodan, or its generic equivalents. The present study was designed to obtain information on the frequency and concentration (in ng/mL) of oxymorphone in the urine of patients prescribed oxycodone.
Over a two-month period (March and April 2007), all 175 patients in a chronic pain practice were asked without advance notice to submit a urine specimen. The patients were being treated for various types of chronic non-cancer pain, with back pain being the most common diagnosis. Eighty-eight patients who were being prescribed oxycodone (extended-release and/or immediate-release) were tested for oxycodone and oxymorphone by an enzyme immunoassay (EIA). Because the usual immunoassay screen for opiates will not pick up oxycodone and oxymorphone, the order was written as “Routine urine drug test plus oxycodone and oxymorphone.” Each patient’s daily dose and time of last dose were recorded by the medical assistant, who also checked the temperature of the urine immediately after voiding to be sure it fell within the range of 90-100 ºF. Two of the patients had UDT results that were negative on immunoassay for oxycodone. They were excluded from the remainder of this study. For 48 of the remaining 86 patients who tested positive for oxycodone on immunoassay, the urine concentration of oxycodone and oxymorphone was determined quantitatively using gas chromatography/mass spectrometry (GC/MS). The cut-off level for a “Positive” oxycodone or oxymorphone result was 100 ng/mL.
|Dose qd||N (total 86)||% of 86|
|Dose qd||N=48||%||Percent oxymorphone/oxycodone|
|15-50mg||17||35.4%||0. 2.5, 3.3,3.5, 4.6, 9.8, 10.0, 10.6, 11.7, 14.3, 15.2, 25.2, 34.1, 36.5, 41.1, 99.5, 138.9|
|51-100mg||11||22.9%||0.67, 7.6, 12.8, 13.2, 14.6, 14.6, 37.3, 71.9, 96.5, 96.9, 147.4|
|101-200mg||7||14.6%||4.8, 5.8, 6.7, 13.7, 18.5, 19.4, 28.5|
|201-300 mg||6||12.5%||10.5, 11.6, 11.6, 19.1, 19.8, 184.1|
|>301 mg||7||14.6%||5.2, 5.5, 10.0, 14.7, 17.8 26.0, 45.8|
|Oxycodone dose qd||oxymorphone (ng/ml)||Oxycodone (ng/ml)||%OM/OC|
Of eighty-six patients whose urines were positive for oxycodone by EIA screen, 80 (93%) were also positive for oxymorphone. The cut-off for the immunoassay screens was 100 ng/ml of oxycodone and 100 ng/ml of oxymorphone. The six patients who were negative for oxymorphone on immunoassay screen were receiving only relatively small doses of oxycodone ranging from 15 to 45 mg per day. Two of these patients were also tested by GC/MS, and one of the two was positive at a low level by this more sensitive test, which showed a urine level of oxymorphone of 66 ng/ml, below the cutoff of 100 ng/mL of the immunoassay. It was also noted that 22 other patients on the same dose range of oxycodone screened positive for both oxycodone and oxymorphone by immunoassay.
Table 1 summarizes the daily doses of oxycodone (sustained-release and/or immediate release) of the 86 patients.