Background <p>Glioblastomas (GBM) are the most aggressive tumors of the nervous system, with therapy limited by multidrug resistance (MDR) and poor drug penetration across the blood-brain barrier (BBB). The BBB expresses ATP-binding cassette (ABC) transporters, particularly ABCB1 (P-glycoprotein, P-gp) and ABCG2 (BCRP), which reduce intracellular drug concentrations and contribute to MDR. The aim of this study was to evaluate a series of β-carboline-quinoline hybrid compounds as potential glioblastoma therapeutics and modulators of ABC transporter activity.</p> Methods <p>Fifteen hybrid compounds, combining harmine and quinoline-based antimalarials via triazole or ureido linkers, were tested for cytotoxicity on U251 glioblastoma and SH-SY5Y neuroblastoma cell lines using cell viability assays. ABC transporter inhibition was assessed with cell-based functional assays measuring intracellular accumulation of fluorescent P-gp and ABCG2 substrates. Selected hybrids were further evaluated for their ability to sensitize chemoresistant cells to the reference drugs doxorubicin (DOXO) and mitoxantrone (MX).</p> Results <p>Most hybrids exhibited stronger cytotoxic effects on glioblastoma cells than their parent compounds and the reference drug temozolomide. Several compounds significantly inhibited P-gp and ABCG2 function, with ureido-type derivatives <b>PU-9</b> and <b>MU-9</b> showing the most pronounced dual inhibition. Importantly, these compounds enhanced the sensitivity of resistant cells to DOXO and MX, suggesting a potential strategy to overcome MDR.</p> Conclusions <p>β-Carboline-quinoline hybrids, particularly <b>PU-9</b> and <b>MU-9</b>, represent promising dual-function agents with both cytotoxic and ABC transporter-modulating activity. Their ability to reduce glioblastoma cell survival and sensitize resistant cells highlights their potential as leads for developing therapeutics that can overcome MDR and improve CNS drug delivery.</p> Clinical trial number <p>Not applicable.</p>