Plus Therapeutics' CNSide Assay: Platform Integration Lacks Data Amidst Guideline Conflicts and Peer Competition

Plus Therapeutics and GTC Partner for NGS Integration into CNSide CSF Assay

Plus Therapeutics, Inc. and Genomic Testing Cooperative (GTC) announced a collaboration to integrate GTC’s next-generation sequencing (NGS) technology into Plus’ CNSide® cerebrospinal fluid (CSF) assay platform. This partnership aims to enable comprehensive DNA and RNA profiling across hundreds of clinically relevant biomarkers from a single CSF specimen. The integration extends CNSide's capabilities from guideline-recognized circulating tumor cell (CTC) enumeration to include DNA and tumor-derived DNA analysis, aligning with the updated NCCN CNS Cancers Guidelines (Version 2.2026). This move is expected to enhance precision diagnostics for metastatic CNS diseases, including leptomeningeal metastases (LM), which affects over 100,000 U.S. patients annually.

• The collaboration significantly expands the CNSide platform's diagnostic power by integrating comprehensive DNA and RNA profiling for hundreds of biomarkers. This directly aligns with the updated NCCN CNS Cancers Guidelines (Version 2.2026), which now recognize CSF circulating tumor cells alongside tumor-derived DNA, broadening the leptomeningeal treatment algorithm to “CSF analysis.” This positions CNSide as a comprehensive CSF-based precision oncology platform.
• By combining hundreds of DNA and RNA biomarkers per CSF specimen with existing CNSide tumor cell and biomarker quantification, the partnership will create a uniquely deep longitudinal molecular data set. This materially enhances the inputs for Plus Therapeutics' AI platform, developed through its strategic partnership with Ephemeral Technologies, enabling more advanced data analytics and insights in CNS oncology.
• The integration is expected to expand CNSide's market and clinical utility across leptomeningeal metastases and other metastatic CNS cancers, supporting broader adoption and higher per-test value. Building on the dedicated AMA PLA code 0640U (effective July 1, 2026) and Medicare enrollment, with commercial payer agreements covering approximately 126 million U.S. lives, the expanded menu will pursue future billing pathways, leveraging existing reimbursement infrastructure.

Addressing the Diagnostic and Treatment Gaps in Leptomeningeal Metastases

Leptomeningeal metastases (LMs) remain one of the most challenging complications in oncology, with current treatment paradigms constrained by both biological barriers and diagnostic limitations. Despite emerging advances in molecular profiling and targeted therapeutics, significant gaps persist in translating these innovations into meaningful survival benefit across the broader patient population.

• Limited efficacy of conventional treatments: Standard approaches—chemotherapy, photon-based radiation therapy, and intrathecal chemotherapy—continue to show restricted therapeutic benefit in LMs.

• Narrow eligibility for advanced therapies: Current options are applicable only to select subpopulations—surgical resection or radiosurgery for patients with a limited number of lesions, targeted therapies for roughly 18% of patients, and immune checkpoint inhibitors, which achieve response rates of only 20–30%.

• Diagnostic constraints: Non-specific clinical presentation combined with the limited sensitivity of conventional CSF cytology hampers timely and accurate diagnosis, delaying initiation of appropriate therapy.

• Biomarker limitations: Issues around sensitivity, specificity, and standardization continue to limit the clinical utility of emerging biomarkers, despite promising data on CSF-based ctDNA variant allele frequency (VAF) analysis and next-generation sequencing (NGS) for disease detection and monitoring.

• Underrepresentation in clinical research: Patients with leptomeningeal disease (LMD) are frequently excluded from clinical trials, restricting their access to investigational agents and slowing evidence generation for this population.

• Persistently poor prognosis: Outcomes remain dismal across tumor types—melanoma-related LMD, for example, carries a median overall survival of just 2.96 months (95% CI: 1.39–4.53)—underscoring the inadequacy of current standards of care.

• Blood-brain barrier (BBB) restriction: The BBB and related CNS barrier mechanisms actively limit the penetration of systemic therapies into the leptomeninges and brain parenchyma, undermining treatment effectiveness even for otherwise active systemic agents.

• Rising incidence amid therapeutic gaps: Improved survival from novel systemic therapies has paradoxically contributed to an increasing incidence of LMs, as these treatments may be less effective in controlling CNS disease—highlighting an urgent unmet need for CNS-penetrant therapeutic strategies.

• Sparse real-world evidence: Contemporary data characterizing prognostic factors and treatment outcomes, particularly in melanoma-associated LMD, remain limited, constraining evidence-based clinical decision-making.

CNSide's Evolution: Integrating NGS for Comprehensive LM Biomarker Profiling

The established gold standard for the diagnosis of leptomeningeal metastases (LM) is the cytological detection of malignant cells in the cerebrospinal fluid (CSF), a method with high specificity but limited sensitivity, ranging from approximately 55% to 83%. This approach is complemented by gadolinium-enhanced magnetic resonance imaging (MRI) of the neuraxis, which demonstrates a sensitivity of around 59-66% and a high specificity of approximately 98%. However, the diagnostic utility of both cytology and conventional imaging is often insufficient for early and definitive detection. Supporting biochemical analyses of the CSF frequently reveal nonspecific abnormalities such as pleocytosis, elevated protein levels, and hypoglycorrhachia, which, while suggestive of LM, lack diagnostic precision on their own.

To address the sensitivity limitations of conventional methods, CSF-based liquid biopsy has emerged as a transformative diagnostic tool. Analysis of circulating tumor cells (CTCs) in the CSF has demonstrated high diagnostic accuracy, with reported sensitivity and specificity of 87.0% and 93.8%, respectively. Even more promising is the analysis of CSF cell-free DNA (cfDNA) using next-generation sequencing (NGS), which has achieved up to 100% sensitivity for diagnosing LM in patients with known primary tumor mutations, far exceeding the performance of cytology and CTC analysis in the same cohorts. This approach is particularly powerful for identifying actionable driver mutations (e.g., in EGFR-mutant NSCLC) and emerging resistance mechanisms (e.g., ALK G1202R in ALK-rearranged NSCLC), with CSF proving to be a more sensitive matrix than plasma for detecting these alterations.

Beyond mutation detection, advanced molecular and imaging techniques are further refining the diagnostic landscape for LM. The detection of genome-wide aneuploidy in CSF-derived cfDNA, using methods like the modified fast aneuploidy screening test-sequencing system (mFAST-SeqS), has been shown to be a successful and prognostically significant biomarker, as the presence of aneuploidy is strongly associated with LM development and poorer overall survival. In parallel, advanced imaging modalities offer complementary insights. For instance, 18F-FDG PET/CT can identify LM-related metabolic activity before anatomical changes are visible, while specific MRI findings, such as unique T2/FLAIR hyperintense brainstem lesions in patients with EGFR-mutant lung adenocarcinoma, can provide crucial diagnostic clues.

Expanding CNSide's Reach and Strategic Impact in LM Precision Oncology

The management of leptomeningeal metastases (LM) is challenged by significant unmet needs, beginning with the condition's rising incidence. As novel systemic therapies improve patient survival, the central nervous system often remains a sanctuary site, leading to more frequent LM diagnoses. Conventional treatments, including systemic chemotherapy and external beam radiation, offer limited efficacy, with median survival often measured in months. Furthermore, established intrathecal therapies carry substantial risks; for example, intra-CSF methotrexate is associated with disseminated necrotizing leukoencephalopathy (DNL), a rare (2.3% incidence) but fatal complication for which risk increases significantly after ten or more rounds of treatment. This landscape is further complicated by a lack of standardized treatment protocols, validated diagnostics, and consensus on clinical trial design and response assessment, hindering the development and evaluation of new therapeutic strategies.

In response to these challenges, recent research has focused on precision oncology approaches targeting well-defined, high-risk patient populations. A primary focus is on non-small-cell lung cancer (NSCLC) with specific driver mutations, particularly EGFR variants. For these patients, strategies combining high-dose third-generation EGFR-TKIs with intrathecal chemotherapy have demonstrated high rates of intracranial symptom relief and disease control. Similarly, patients with HER2-positive breast cancer and LM are being targeted with regimens like tucatinib-trastuzumab-capecitabine, which has shown a median overall survival of 10 months versus historical controls. Other key populations under investigation include those with triple-negative breast cancer, melanoma, and IDH-wildtype glioblastoma, with research identifying specific prognostic factors and high-risk features within these groups.

The drive for more effective and less toxic treatments has spurred the investigation of novel therapeutic modalities. Antibody-drug conjugates (ADCs) like trastuzumab deruxtecan are showing activity in HER2-altered breast cancers with LM. In the preclinical setting, targeted radiopharmaceutical therapy with agents such as 225Ac-NM600 has demonstrated superior anti-tumor efficacy and reduced off-target toxicity compared to external beam radiation in LM models of breast and lung cancer. Concurrently, efforts are underway to overcome barriers to immunotherapy, as patients relapsing with isolated LM often lack the extracranial tumor tissue required for manufacturing cellular therapies like tumor-infiltrating lymphocytes (TILs). These targeted and innovative strategies represent a critical shift in the paradigm for managing this difficult-to-treat disease.

Unlocking Precision in CNS Cancer: The Power of CSF Multi-Omic Analysis

The landscape of central nervous system (CNS) cancer diagnostics is undergoing a transformative shift, driven by the urgent need for more precise and less invasive methods to manage devastating conditions like leptomeningeal metastases (LM). Historically, diagnosing LM has been challenging, relying on cerebrospinal fluid (CSF) cytology and MRI, which often lack sensitivity and can delay critical treatment decisions. However, the emergence of liquid biopsies, particularly the analysis of CSF, is revolutionizing this paradigm.

This recent collaboration to integrate next-generation sequencing (NGS) into the CNSide® CSF assay platform represents a pivotal step forward. By enabling comprehensive DNA and RNA profiling from a single CSF specimen, the enhanced platform moves beyond simple circulating tumor cell (CTC) enumeration to offer a detailed molecular blueprint of CNS tumors. Research consistently demonstrates that CSF-derived circulating tumor DNA (ctDNA) provides a more accurate and comprehensive representation of brain tumor genomic alterations than plasma, allowing for the identification of actionable mutations that can guide personalized therapies. This capability is crucial for patients with CNS metastases, where targeted treatments and immunotherapies are increasingly showing promise.

While the potential is immense, several considerations remain. The success of NGS relies on sufficient DNA yield, and some studies have noted challenges with low DNA concentrations in CSF, which could impact the assay's reliability. Furthermore, the inherent heterogeneity of tumor cells and DNA within the CSF suggests that a multi-omic approach, combining various analytes, may be necessary to capture the full picture. Despite these challenges, the alignment of this advanced platform with updated NCCN CNS Cancers Guidelines underscores its growing clinical relevance. This development promises to empower clinicians with real-time genomic insights, facilitating earlier diagnosis, more precise treatment selection, and dynamic monitoring of disease progression and resistance, ultimately aiming to improve outcomes for patients facing these aggressive cancers.