CSF Drug Measurement in 20 Lung Cancer Patients with Brain Metastases

A single-center retrospective study of 20 patients with EGFR-mutant non-small cell lung cancer and brain metastases found that higher cerebrospinal fluid exposure to almonertinib was associated with better brain-MRI response, while no statistically significant exposure-response differences were detected among response groups treated with osimertinib. Researchers at the Fourth Hospital of Hebei Medical University in China developed and validated an ultra-performance liquid chromatography-tandem mass spectrometry method to measure both drugs simultaneously in matched plasma and cerebrospinal fluid samples. Seven patients received osimertinib and 13 received almonertinib. The results indicate that plasma trough concentration alone did not capture the marked variation in central nervous system drug exposure, but the small observational study cannot establish monitoring thresholds or show that cerebrospinal fluid-guided treatment improves outcomes.

The clinical samples were collected synchronously in the morning before the patients’ daily doses, providing paired measurements at steady-state trough concentration. The investigators recorded plasma and cerebrospinal fluid concentrations for each participant and calculated a blood-brain barrier penetration rate as cerebrospinal fluid concentration divided by plasma concentration, multiplied by 100%. Chest CT and brain MRI acquired before and after a defined treatment course were evaluated under Response Evaluation Criteria in Solid Tumors, with response classified as complete response, partial response, stable disease or progressive disease. Because participants received one of the two drugs rather than being randomized to treatment or measured under a crossover design, the analysis could compare exposure patterns within each group but could not isolate the effects of the drugs from differences between patient groups.

The assay used a C18 column maintained at 40 degrees Celsius, with water containing 0.1% formic acid and acetonitrile as mobile phases, and a flow rate of 0.3 mL per minute. Tandem mass spectrometry operated in positive-ion multiple reaction monitoring mode, monitoring transitions of 500.2 to 72.1 for osimertinib, 506.1 to 72.2 for almonertinib and 491.2 to 352.0 for the internal standard glimepiride. Samples underwent protein precipitation with methanol followed by dilution in 50% aqueous methanol. Calibration ranges were 1 to 1,000 ng/mL in plasma and 1 to 100 ng/mL in cerebrospinal fluid for both drugs, with a lower limit of quantification of 1 ng/mL in each matrix. Weighted linear regression yielded coefficients of determination from 0.994 to 0.998.

Selectivity testing found that responses from interfering components at the analytes’ retention times were less than 20% of the response at the lower limit of quantification and less than 5% of the internal-standard response. Intra-day and inter-day precision, expressed as relative standard deviation, was no more than 15% for quality-control samples, while accuracy remained within ±10% of nominal concentrations. Matrix effects ranged from 86.89% to 104.23%, with relative standard deviations below 10%, indicating that plasma and cerebrospinal fluid components did not materially distort measurement under the validated conditions. Mean extraction recoveries were approximately 103%. Both analytes also remained within the study’s stability limits after six hours at 25 degrees Celsius, 30 days at −80 degrees Celsius and four freeze-thaw cycles.

Among the seven osimertinib-treated patients, the mean plasma concentration was 179.46 ng/mL, with a range of 45.30 to 395.87 ng/mL. Mean cerebrospinal fluid concentration was 6.35 ng/mL, ranging from 1.54 to 24.88 ng/mL, and the mean penetration rate was 3.23%, with individual values from 1.21% to 6.29%. Two patients had partial responses in brain metastases, three had stable disease in both lung and brain, and two had partial responses in lung lesions with stable intracranial disease. The patients with partial brain responses had cerebrospinal fluid concentrations of 7.8 and 24.9 ng/mL and penetration ratios of 3.24% and 6.29%, respectively. However, the small group showed no statistically significant differences in cerebrospinal fluid concentration or penetration rate among brain-response categories, and steady-state plasma concentration was not significantly associated with intracranial response.

The 13 almonertinib-treated patients had a mean plasma concentration of 182.19 ng/mL, ranging from 82.40 to 343.23 ng/mL. Their mean cerebrospinal fluid concentration was 8.39 ng/mL, ranging from 0.64 to 31.13 ng/mL, while the mean penetration rate was 5.92%, with a substantially wider individual range of 0.34% to 30.86%. Brain MRI outcomes consisted of partial response in three patients, stable disease in seven and progressive disease in three. Those with partial response had significantly higher cerebrospinal fluid concentrations than patients with stable or progressive disease, with a reported P value below 0.001, and higher penetration ratios, with P below 0.01. No significant difference in either cerebrospinal fluid concentration or penetration rate was detected between the stable- and progressive-disease groups. Plasma concentration was not significantly correlated with lung CT response in this group.

Individual cases illustrated how systemic and intracranial outcomes could differ. One almonertinib recipient achieved partial response in lung lesions but had progressive disease in the brain; this patient’s plasma concentration was 249 ng/mL, compared with a cerebrospinal fluid concentration of 1.1 ng/mL and a penetration rate of 0.44%. Another patient had a cerebrospinal fluid concentration of 0.71 ng/mL and a penetration rate of 0.59% while developing progressive meningeal metastasis. By contrast, three almonertinib-treated patients achieved partial brain responses and had cerebrospinal fluid concentrations of 30.04, 31.13 and 29.83 ng/mL, corresponding to penetration ratios of 11.20%, 30.86% and 14.89%. These observations are consistent with the statistical association between central nervous system exposure and intracranial response in the almonertinib group, but they do not establish that exposure alone caused the different outcomes.

Although the two groups had similar mean trough plasma concentrations, their cerebrospinal fluid distributions differed. Mean almonertinib exposure in cerebrospinal fluid was 8.39 ng/mL, compared with 6.35 ng/mL for osimertinib, and mean penetration was 5.92% and 3.23%, respectively. Because the groups were small, non-randomized and composed of different patients, those comparisons cannot demonstrate that one drug is universally superior for brain metastases. The authors noted that almonertinib has lower reported affinity for P-glycoprotein and less active efflux from the central nervous system, whereas osimertinib is a moderate P-glycoprotein substrate, but transporter expression or activity was not measured in patient brain tissue. Substantial individual variation in penetration was therefore not explained mechanistically in this study.

Important limitations constrain the clinical interpretation. The analysis included only 20 participants at one center and used short-term imaging response rather than long-term survival or intracranial progression-free survival. Cerebrospinal fluid was sampled at a single fixed steady-state point, so the analysis could not capture changes in drug penetration during treatment or determine whether exposure was sustained over time. Brain-tissue measurements of the efflux transporters P-glycoprotein and BCRP were absent, leaving possible biological reasons for variable permeability unresolved. The osimertinib subgroup was particularly small, and its lack of significant exposure-response differences means the combined analysis should not be described as proof of the same relationship for both drugs.

Overall, the work combines a validated bioanalytical method with paired clinical measurements and imaging outcomes. Its clearest clinical observation is that cerebrospinal fluid concentrations and penetration rates varied widely among patients receiving almonertinib, and higher central nervous system exposure was associated with better brain-MRI response in that group. The osimertinib results were directionally suggestive in several cases but were not statistically significant. The findings provide preliminary support for studying cerebrospinal fluid therapeutic drug monitoring as part of individualized management of EGFR-mutant lung cancer with brain metastases, while larger prospective multicenter studies are needed to establish reliable concentration thresholds, assess survival outcomes and test whether monitoring-guided dose adjustment improves intracranial disease control.

Subject of Research: Simultaneous measurement of osimertinib and almonertinib in plasma and cerebrospinal fluid and association with brain-metastasis response in 20 patients

Article Title: Simultaneous determination of osimertinib and almonertinib concentrations in human plasma and cerebrospinal fluid using UPLC-MS/MS method and its clinical applications

Article References: Yin, J., Zuo, Y., Wang, X., Zhang, Z., Yue, H., Shi, J., Han, Y., Liu, J., & Liang, P. (2026). Simultaneous determination of osimertinib and almonertinib concentrations in human plasma and cerebrospinal fluid using UPLC-MS/MS method and its clinical applications. Frontiers in Pharmacology, 17, Article 1943759. https://doi.org/10.3389/fphar.2026.1943759

Image Credits: AI Generated

DOI: 10.3389/fphar.2026.1943759

Keywords: osimertinib, almonertinib, cerebrospinal fluid, EGFR inhibitors, brain metastases, non-small cell lung cancer, UPLC-MS/MS, blood-brain barrier, therapeutic drug monitoring, drug penetration, Simultaneous, determination