TAK-285, an investigational, orally active HER2/EGRF inhibitor is in clinical development for potential use in HER2 over-expressing metastatic breast cancer. The objective of the present work was to verify the presence of unbound TAK-285 in the rat brain after oral administration by a microdialysis technique with simultaneous sampling of blood and brain. In a pilot microdialysis experiment no detectable amount of TAK-285 was found in the brain dialysate samples after oral administration of the drug (50 mg/kg). A conventional pharmacokinetic study was performed simultaneously with the pilot microdialysis study using the same dosing suspension. TAK-285 was detected in the brain even at the last time point when the samples were taken from the animal at the end-point of the microdialysis experiment. The apparent absence of TAK-285 in blood and brain dialysate samples might be explained by a very low recovery of microdialysis probes for TAK-285 and/or by the adsorption of the compound to the outflow tubing of the microdialysis probes. Results of an in vitro recovery study with TAK-285 were indicative of the strong adsorption of the compound to the microdialysis tubings. Adding bovine serum albumin (4%, w/v) in perfusion fluids and reducing perfusion flow rate (from 1.0 μL/min to 0.5 μL/min) in in vitro experiments substantially improved the detectability of TAK-285 in dialysate samples. Application of new perfusion conditions resulted in a manifold increase of the relative recovery of the microdialysis set-up for TAK-285 (from 1.6% to 47%). Subsequent in vivo microdialysis experiments were performed using the modified perfusion conditions in animals dosed with TAK-285 (75 mg/kg, p.o.). Detectable level of unbound TAK-285 was found in the extracellular space in the brain as long as 24-28 h after administration of the drug. The brain-to-blood ratios of the unbound TAK-285 were 0.18 and 0.24 (calculated from the C(max) values or from the area under the curve [AUC] values) similarly to the brain-to-blood ratios of total TAK-285. On the basis of substantial brain penetration of unbound TAK-285, it is concluded that TAK-285 might have the potential in the treatment of brain metastases of HER2 over-expressing metastatic breast cancer. The methodological approach described here might help to solve similar problems in determination of brain penetration of other substances with strong adsorption to the tubing of microdialysis setups.