The My5‑FU assay (Saladax Biomedical Inc.), previously known as OnDose, is a CE‑marked in vitro diagnostic test designed to measure the levels of 5‑fluorouracil (5‑FU) chemotherapy in plasma samples. The assay is intended for use in people who are having 5‑FU chemotherapy by continuous infusion, to allow pharmacokinetic dose adjustment and therapeutic drug monitoring, with the aim of achieving an optimal plasma level of 5‑FU.
The My5‑FU assay is a homogenous 2‑reagent nanoparticle agglutination assay that can be adapted for use on several different clinical chemistry analysers. The assay needs plasma from a peripheral venous blood sample, taken towards the end of each 5‑FU infusion cycle using an EDTA or heparin tube. The assay is based on the principle of measuring scattered light; when higher levels of 5‑FU are present in the plasma sample, less light is scattered. The assay typically takes about 10 to 15 minutes for the first results, depending on the analyser used, with subsequent results taking less than a minute. Results are reported in nanograms of 5‑FU per millilitre of plasma and are converted to an area under the (concentration) curve value. Values of greater than 50 milligram hours per litre may signify that the blood sample has been taken too close to the infusion port and may need to be disregarded. The assay has a limit of detection of 52 nanograms/ml and a lower limit of quantitation of 85 nanograms/ml.
When using the My5‑FU assay in clinical practice, the initial dose of 5‑FU is calculated according to the patient's body surface area. A sample of the patient's blood is taken towards the end of the infusion cycle, at least 18 hours after the start of the infusion, while the pump is infusing at a steady rate. Subsequent doses of 5‑FU are then calculated using the area under the curve result from the My5‑FU assay, in conjunction with a validated dose adjustment algorithm. The My5‑FU assay is intended as an aid for managing dose adjustment in patients having 5‑FU chemotherapy and the results of the test should be used in conjunction with clinical assessment to determine whether dose adjustment is appropriate.
The comparator used in this assessment is body surface area dosing. Body surface area is calculated by formulae that use the patient's height and weight, and correlates with blood volume, cardiac output and renal function, all of which influence drug elimination. Usually the dose is calculated according to the patient's actual body weight, unless obesity, oedema or some other form of abnormal fluid retention such as ascites is present. In this case, ideal weight is used as the basis for the calculation. The dose may be adjusted to take into account a patient's liver and kidney function, both of which may impact on how 5‑FU is metabolised and excreted. A 5‑FU dose may also be adjusted according to the severity of any side effects that a patient experiences.