Background Patients with HER2-positive breast cancer (HER2+BC) develop central nervous system metastases twice as often as patients with luminal HER2-negative breast cancer. Leptomeningeal progression results in a drastically altered prognosis with limited therapeutic options. This phase II study was conducted in order to assess the efficacy of intrathecal (IT) trastuzumab in HER2+BC patients with leptomeningeal metastasis (LM), based on a phase I dose-escalation study that had determined the recommended weekly dose of 150 mg for phase II. Methods Eligible patients received weekly administrations of 150 mg of IT trastuzumab. The primary endpoint was clinical neurologic progression-free survival (LM-related-PFS) after 8 weeks. Overall survival (OS), toxicities and quality of life (QoL) were secondary endpoints. Results Among the 19 enrolled patients, 16 (84%) had concomitant brain metastases, 15 of them had received prior radiotherapy to the brain. All patients had received at least one line of systemic anti-HER2 therapy. After 8 weeks, 14 patients (74%) were free of neurological progression. The median LM-related-PFS was 5.9 months and the median OS was 7.9 months. According to the QLQ-C30 and BN20 scales, the global health-related QoL status seemed preserved and no toxicity above grade 3 was observed. Conclusions Conducting studies on patients with LM poses significant challenges and ethical dilemmas inherent to this population. Despite some limits, this phase II study’s findings in terms of clinical neurologic response and quality of life’s control confirms weekly administration of 150 mg of IT trastuzumab as a valuable option for HER+BC patients with LM and support the interest for further investigations.

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Intravenous administration of monoclonal antibodies leads to low concentrations in the central nervous system, which is a serious concern in neuro‐oncology, especially in leptomeningeal carcinomatosis of HER2‐overexpressing breast cancer. Case reports of i.t. administrations of trastuzumab have shown promising results in these patients but dosing regimens are empirical in absence of pharmacokinetic (PK) study. With a population PK approach, we described the fate of trastuzumab after i.t. administration in 21 women included in a phase I–II clinical trial. Trastuzumab was administered by i.t. route every week for 8 weeks and both cerebrospinal fluid (CSF) and serum were sampled to measure trough concentrations. Some patients showed noticeable CSF concentration fluctuations predicted using a target‐mediated drug disposition. This target was latent and produced with a delayed feedback. Apparent volumes of distribution were close to physiological volumes (V1 = 3.25 L, V2 = 0.644 L, for serum and CSF, respectively). Estimated (constant) transfer from serum to CSF was very slow (k12 = 0.264 mg/day) whereas estimated half‐life of transfer from CSF to serum was rapid (2.2 days). From the individual parameters of patients, a single i.t. administration of 150 mg of trastuzumab corresponded to median mean residence times of 3.8 days and 15.6 days in CSF and serum, respectively. Survival without neurological relapse was not related to trastuzumab exposure. This study confirms that transfer of trastuzumab from serum to CSF is very limited and that this monoclonal antibody, when administered by i.t. route, is rapidly transferred to the serum.

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