Abstract: Volasertib (BI6727) is a highly potent, selective, small-molecule ATP-competitive inhibitor of Polo-like kinase 1 (PLK1), a master regulator of mitotic progression. While extensively studied in adult acute myeloid leukemia (AML) and solid tumors, volasertib has emerged as a promising targeted therapeutic agent for pediatric malignancies, particularly rhabdomyosarcoma (RMS), acute lymphoblastic leukemia (ALL), and Ewing sarcoma. High PLK1 expression is correlated with poor prognosis in these pediatric cancers. Preclinical models demonstrate that volasertib induces a distinct mitotic arrest ("Polo arrest") and subsequent apoptosis, and uniquely downregulates the oncogenic PAX3-FOXO1 fusion protein in RMS. Despite its potent single-agent activity, clinical translation is limited by dose-dependent hematological toxicities and the potential for acquired resistance. Consequently, current research emphasizes rational combination strategies, such as pairing volasertib with microtubule-destabilizing agents like vincristine, which have shown synergistic efficacy in pediatric models. This review synthesizes the pharmacological activity, molecular mechanisms, structure-activity relationships, limitations, and future perspectives of volasertib in the context of pediatric oncology.
1. Introduction
The Polo-like kinases (PLKs) comprise a family of serine/threonine protein kinases that play integral roles in cell cycle regulation, including entry into mitosis, centrosome maturation, spindle assembly, sister chromatid separation, and cytokinesis [1][2]. Among them, PLK1 is the most extensively characterized and is frequently overexpressed in a wide variety of malignancies, including pediatric solid tumors and leukemias [3]. In pediatric cancers such as rhabdomyosarcoma (RMS), high PLK1 expression correlates with poor event-free and overall survival, making it an attractive therapeutic target [3].
Volasertib (BI6727) is a highly selective, second-generation PLK inhibitor developed to overcome the pharmacokinetic limitations of its predecessor, BI 2536 [1]. By providing sustained exposure to tumor tissues and demonstrating a manageable safety profile in early trials, volasertib has advanced significantly in clinical development [1]. While much of its clinical evaluation has focused on adult acute myeloid leukemia (AML), emerging preclinical and early-phase clinical data highlight its substantial potential in treating pediatric malignancies [3].
2. Pharmacological Activity
Volasertib has demonstrated potent anti-proliferative activity across a spectrum of pediatric cancer models. In vitro studies have shown that pediatric cancer cell lines—including acute lymphoblastic leukemia (ALL), medulloblastoma, osteosarcoma, Ewing sarcoma, neuroblastoma, and RMS—exhibit variable but significant sensitivities to PLK1 inhibition, with half-maximal growth inhibitory concentrations (GI50) ranging from 4 to 40 nM [3]. Notably, PAX3-FOXO1 fusion-positive RMS cell lines display particularly low GI50 values, indicating high sensitivity [3]. Volasertib has also been shown to inhibit the growth and survival of cell lines derived from patients with pediatric ALL [1].
In vivo, volasertib monotherapy has induced complete and partial tumor regressions in established xenograft models of fusion-positive RMS (e.g., RMS-01 and RH30R) [3]. Furthermore, volasertib exhibits strong synergistic pharmacological activity when combined with other chemotherapeutic agents. In RMS models, volasertib is highly synergistic with microtubule-destabilizing drugs such as vincristine, vinblastine, and vinorelbine [3]. Similarly, in Ewing sarcoma cell lines, the combination of volasertib and eribulin significantly enhanced DNA fragmentation and suppressed colony formation [6]. Clinically, volasertib has successfully completed a Phase I dose-escalation study as a single agent in pediatric patients with leukemia and refractory solid tumors, identifying a recommended Phase II dose (RP2D) that is higher than the RP2D established for adult patients [3].
3. Molecular Mechanism of Action
Volasertib functions as an ATP-competitive kinase inhibitor that selectively targets PLK1. By inhibiting PLK1, volasertib disrupts mitotic spindle assembly, leading to a distinct cellular phenotype known as "Polo arrest" [1][5]. This state is characterized by cell cycle arrest in prometaphase, the accumulation of phospho-histone H3, and the formation of aberrant monopolar mitotic spindles where kinetochores are improperly attached, ultimately culminating in apoptosis [1][2].
In the specific context of pediatric RMS, PLK1 inhibition by volasertib exerts additional molecular effects beyond mitotic arrest. Treatment of fusion-positive RMS cells with volasertib leads to the degradation of the oncogenic PAX3-FOXO1 fusion protein and a subsequent reduction in the transcription of its downstream targets, such as AP2beta [3]. Furthermore, volasertib induces DNA damage, activating the ATM-CHK1/CHK2 checkpoint pathway [3]. When combined with vincristine, volasertib triggers mitochondrial apoptosis via the inactivation of anti-apoptotic BCL2 family proteins, specifically Myeloid Cell Leukemia-1 (MCL-1), activating both caspase-dependent and independent cell death pathways [3].
4. Structure-Activity Relationship (SAR)
Volasertib belongs to the dihydropteridinone class of small-molecule compounds [2][5]. It was rationally designed and tailored from its predecessor, BI 2536, to optimize potency, kinase selectivity, and pharmacokinetic efficacy [1]. Structurally, volasertib binds to the ATP-binding pocket of PLK1 at the hinge region between the amino-terminal and carboxyl-terminal lobes of the kinase domain. This binding is stabilized through hydrogen bonds extending from the dihydropteridinone core to the backbone amino and carbonyl groups of the Cys133 residue [5].
Because the ATP-binding pocket is highly conserved among PLK family members, volasertib also inhibits PLK2 and PLK3, though with significantly lower affinity. The half-maximal inhibitory concentrations (IC50) are 0.87 nM for PLK1, 5.0 nM for PLK2, and 56 nM for PLK3 [2][5]. It exhibits exceptional selectivity, showing no appreciable inhibition against a panel of over 60 other unrelated kinases at concentrations up to 10 μM [1][2]. This structural optimization also confers a superior pharmacokinetic profile, characterized by a large volume of distribution (indicating excellent tissue penetration), moderate clearance, and a long terminal half-life (approximately 111 to 135 hours in humans) [1][2].
5. Current Limitations
Despite its potent preclinical activity, the clinical application of volasertib faces several limitations. First, single-agent PLK1 inhibition is unlikely to be curative in pediatric solid tumors like RMS. Preclinical models have demonstrated tumor re-growth following volasertib monotherapy, indicating innate or emerging resistance, potentially driven by the tumor evolving independence from fusion proteins [3]. Additionally, the high doses required to achieve complete tumor regression in some murine xenograft models may exceed the maximum tolerated doses in humans [3].
Toxicity remains a significant clinical hurdle. The dose-limiting toxicities (DLTs) of volasertib are predominantly hematological, including reversible thrombocytopenia, neutropenia, and febrile neutropenia, which are consistent across adult and pediatric populations [1][2][3]. Furthermore, combination therapies must be carefully scheduled to avoid drug antagonism. For instance, combining volasertib with topoisomerase II inhibitors (like etoposide or doxorubicin) can be antagonistic if administered concurrently, because these agents induce G2 cell cycle arrest, preventing cells from entering mitosis where volasertib exerts its primary "Polo arrest" effect [3].
6. Future Perspectives
The future of volasertib in pediatric oncology relies heavily on rational combination strategies. The synergistic relationship between volasertib and vincristine—a staple in the standard first-line and relapse treatment of RMS—holds immediate clinical promise for both fusion-positive and fusion-negative RMS [3]. Other biologically driven combinations are also under investigation, such as pairing volasertib with BET inhibitors (e.g., BI894999) to further suppress PAX3-FOXO1 transcriptional activity, or with BH3 mimetics (e.g., navitoclax) to exploit apoptotic vulnerabilities mediated by proteins like NOXA and BCL-XL [3].
To improve clinical translation, future preclinical testing must utilize advanced model systems that better recapitulate primary pediatric tumors, such as 3D in vitro models and patient-derived xenografts (PDXs), rather than relying solely on long-established cell lines that may artificially overexpress PLK1 [3]. Finally, the identification of predictive biomarkers—such as specific PLK1-regulated phosphoproteins, p53 functional status, or genetic signatures—will be crucial to identify pediatric patients most likely to benefit from volasertib therapy and to optimize personalized treatment regimens [1].
