HPLC for heroin replacements: Opioid therapy monitored with coulometric detection

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  • Published: Sep 5, 2011
  • Author: Steve Down
  • Channels: HPLC
thumbnail image: HPLC for heroin replacements: Opioid therapy monitored with coulometric detection

Heroin replacement therapy

The classic approach for weaning heroin adducts off the drug is opioid replacement therapy, in which synthetic opioids are used to block the effects of heroin. Cravings are reduced and withdrawal symptoms are blocked, so that the addict does not feel compelled to take more heroin.

The importance of preventing withdrawal cannot be emphasised enough. The symptoms are so debilitating that an addict can think that the only way to appease them is to take more heroin. Severe stomach pain, diarrhoea, nausea, vomiting, fever, limb aches and pains and sleep problems are just some of the trials to face.

The most popular replacement drug is methadone, which can stabilise and relieve patients for up to 60 hours. In the long term, the normal plan is to reduce the dose slowly until the patient is clear of all opiates and can return to a normal life. However, there is the risk of becoming tolerant to methadone as well as physically dependent, and overdosing is not uncommon.

A more recent, safer alternative is buprenorphine, which is metabolised in the body to the active agent norbuprenorphine. It produces only mild withdrawal symptoms and a low risk of abuse when it is taken sublingually. Unfortunately, the drug community soon discovered that the drug can be abused if it is taken intravenously, so a mixed formulation containing buprenorphine and naloxone was introduced to avoid the possibility.

Naloxone has very poor sublingual availability and does not interfere with buprenorphine. But when taken intravenously, the drug becomes available and blocks the opioid effects of buprenorphine to initiate opiate withdrawal.

In drug testing labs, various methods have been introduced to measure the three opioid replacement drugs methadone, naloxone and buprenorphine and its active metabolite, which help to monitor ongoing therapy and switches between treatment regimes.

Now, scientists in Europe have devised a method involving HPLC with coulometric detection, which they claim will be cheaper to run than existing methods while maintaining good selectivity and sensitivity.

Maria Augusta Raggi and colleagues from the University of Bologna, the Addiction Treatment Centre, Cossato, Italy, and the Drug Prevention and Health Branch of the UN Office on Drugs and Crime, Vienna, combined the method with a novel microextraction technique.

Electrolytic examination of opioids

The method was developed with standard solutions, initially adopting a published procedure for analysis of the three drugs by HPLC with diode array detection. Using a C8 column and a phosphate buffer-acetonitrile-methanol mobile phase, separation of the drugs was not good enough, with the naloxone peak too close to the void peak.

Subsequent variation of the column type and the mobile phase compositions led to satisfactory separation on a cyanopropyl column with a mobile phase comprising acetonitrile and mildly acidic phosphate buffer at pH 6.4. Under these conditions, the four analytes and levosulpiride, added as an internal standard, were well separated within 15 minutes.

The mobile phase pH had to be higher than 5.0 for the compounds to be oxidised during coulometric detection. An oxidation potential of 0.600 V was employed at the first detector in the analytical cell for the analytes, while a reduction potential of -0.200 V was applied to the second detector to successfully eliminate biological interference.

The good separation was maintained when the drugs were added to blank plasma, the chosen fluid, the lack of interferences from the endogenous plasma components being confirmed.

The detection limits were 0.08 ng/mL for buprenorphine and norbuprenorphine, 0.9 mg/mL for methadone and 0.04 ng/mL for naloxone.

A second key step was the development of a novel procedure for extracting the drugs from plasma, involving microextraction by packed sorbent (MEPS). A small amount of C8 sorbent was inserted into a glass syringe and activated with acetonitrile then water.

Plasma was spiked with the internal standard, diluted with water and drawn through the syringe 15 times without being discarded. After a washing step, the analytes were eluted with acetonitrile and reconstituted in the HPLC mobile phase for analysis.

This method greatly reduced the volumes of sample and solvents to microlitres rather than millilitres compared with conventional SPE and was quicker and cheaper. The extraction yields for the analytes were greater than 85%, so performance was not compromised.

Therapy monitoring in recovering addicts

The procedure was applied to blood samples taken from two recovering heroin addicts on opioid replacement therapy with Suboxone (a combination of buprenorphine and naloxone) or methadone.

In both cases, the analytes were detected without interferences and the measured concentrations were within the therapeutic ranges of 0.5-10 ng/mL for buprenorphine and 40-1000 ng/mL for methadone. Both patients were considered responders to their therapy, there being no evidence of continued illicit opiate use in the chromatograms.

The procedure was proven to be robust by incurred sample reanalysis, all of the measurement criteria being within the accepted limits of ± 20%.

The combination of MEPS and HPLC with coulometric detection is a faster, cheaper alternative to existing procedures for the measurement of the opioid replacement drugs and could be used to monitor the progress of ex-heroin addicts undergoing treatment.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.


A series of opioids used in drug replacement therapy for heroin adducts have been determined in plasma by HPLC with coulometric detection in the first procedure for simultaneously measuring methadone, naloxone, buprenophine and its primary metabolite

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