Abstract: Multiple myeloma (MM) is an incurable hematologic malignancy that eventually relapses despite the use of standard therapies such as proteasome inhibitors, immunomodulatory drugs, and monoclonal antibodies. Selinexor (KPT-330) is a first-in-class, orally bioavailable selective inhibitor of nuclear export (SINE) compound that specifically targets exportin-1 (XPO1). By blocking XPO1, selinexor forces the nuclear retention and reactivation of tumor suppressor proteins and glucocorticoid receptors, while preventing the translation of key oncoproteins. Clinical trials, notably STORM and BOSTON, have demonstrated significant efficacy in heavily pretreated, relapsed/refractory multiple myeloma (RRMM) patients, leading to its regulatory approval. Despite its robust pharmacological activity and synergistic potential with other anti-myeloma agents, selinexor's clinical utility is challenged by dose-dependent hematological and gastrointestinal toxicities. Current research focuses on optimizing dosing schedules, implementing aggressive supportive care, and exploring novel combinations—including immunotherapies and CAR-T cell bridging—to maximize efficacy and tolerability. Furthermore, second-generation SINE compounds are under investigation to widen the therapeutic window.
1. Introduction
The treatment landscape for multiple myeloma (MM) has been revolutionized over the past two decades by the introduction of proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs), and monoclonal antibodies (mAbs) [4]. Despite these advances, MM remains an incurable disease. Patients inevitably relapse and often become refractory to multiple drug classes, leading to the emergence of "triple-class refractory" or "penta-refractory" disease states with poor prognoses and limited therapeutic options [1][5].
To address this unmet clinical need, selinexor (KPT-330) was developed as a novel, oral, first-in-class selective inhibitor of nuclear export (SINE) compound [1]. In 2019, the United States Food and Drug Administration (FDA) granted accelerated approval to selinexor in combination with dexamethasone for adult patients with relapsed or refractory multiple myeloma (RRMM) who have received at least four prior therapies and whose disease is refractory to at least two PIs, two IMiDs, and an anti-CD38 monoclonal antibody [1][4]. Subsequently, based on the phase III BOSTON trial, selinexor was approved in combination with once-weekly bortezomib and dexamethasone for MM patients with at least one prior line of therapy [4].
2. Pharmacological Activity
Selinexor has demonstrated potent anti-myeloma activity both in vitro and in vivo, particularly when used in combination regimens [1]. The pivotal phase IIb STORM trial evaluated selinexor plus dexamethasone in heavily pretreated, penta-exposed, and triple-class refractory RRMM patients. The overall response rate (ORR) was 26.2%, with a median progression-free survival (PFS) of 3.7 months and a median overall survival (OS) of 8.6 months [1][4].
In earlier lines of therapy, the phase III BOSTON trial compared once-weekly selinexor, bortezomib, and dexamethasone (SVd) against standard twice-weekly bortezomib and dexamethasone (Vd). The SVd regimen significantly improved the ORR (76.4% vs. 62.3%) and extended the median PFS (13.93 months vs. 9.46 months) [4][5]. Furthermore, the multi-arm STOMP trial has shown that selinexor exhibits marked synergy with various backbone treatments. For instance, selinexor combined with carfilzomib and dexamethasone (SKd) yielded an ORR of 78% in heavily pretreated patients [1][4]. Combinations with pomalidomide (SPd), lenalidomide (SRd), and daratumumab (SDd) have also demonstrated high response rates, confirming selinexor's ability to overcome acquired resistance to standard agents [4][5].
3. Molecular Mechanism of Action
Exportin-1 (XPO1), also known as chromosomal region maintenance 1 (CRM1), is a key karyopherin protein responsible for the nuclear export of over 200 cargo proteins. XPO1 is overexpressed in MM and other malignancies, correlating with advanced disease, drug resistance, and poor survival [1][4]. Elevated XPO1 activity leads to the aberrant cytoplasmic accumulation and functional inactivation of major tumor suppressor proteins (TSPs) such as p53, p21, p27, FOXO3A, and retinoblastoma (RB) [3][4].
Selinexor specifically blocks XPO1, preventing the nuclear export of these TSPs. This blockade forces the nuclear retention and reactivation of TSPs, leading to cell cycle arrest and apoptosis in malignant cells [1]. Additionally, XPO1 inhibition traps the messenger RNA of key oncoproteins (such as c-Myc and Bcl-2) in the nucleus, preventing their translation in the cytoplasm [1][11]. Selinexor also retains activated glucocorticoid receptors (GRs) in the nucleus, which synergizes with dexamethasone therapy [4]. Furthermore, selinexor prevents the export of IκB (the inhibitor of NF-κB), leading to its nuclear accumulation and the subsequent suppression of NF-κB transcriptional activity, a mechanism that helps overcome resistance to proteasome inhibitors [3][4].
4. Structure-Activity Relationship (SAR)
Selinexor belongs to the SINE family of small molecules, which are designed with N-azolylacrylate derivatives [4]. The compound exerts its effect by binding covalently and reversibly to the cysteine-528 (Cys528) residue located within the hydrophobic cargo-binding pocket of XPO1 [4].
A critical aspect of selinexor's SAR is its slowly reversible binding kinetics. Older nuclear export inhibitors, such as leptomycin B, bound irreversibly to XPO1, causing permanent blockade of nuclear export in both malignant and healthy cells, which resulted in severe, dose-limiting systemic toxicities (e.g., profound anorexia and malaise) [3]. In contrast, selinexor's transient target inhibition allows the drug to gradually disengage from XPO1 after withdrawal. This dynamic binding profile enables normal cells to recover and resume normal nuclear export processes, thereby providing a viable therapeutic window for clinical use [3][4]. To further optimize this profile, second-generation SINE compounds like eltanexor (KPT-8602) have been rationally designed with reduced blood-brain barrier penetration to limit central nervous system toxicities while maintaining potent anti-tumor activity [3][7].
5. Current Limitations
The clinical application of selinexor is primarily limited by its toxicity profile, which requires careful dose management and aggressive supportive care. The most frequent adverse events are hematological and gastrointestinal [1][4].
Hematological toxicities include severe thrombocytopenia, anemia, and neutropenia. Thrombocytopenia is dose-dependent and occurs in up to 73% of patients (with significant Grade 3/4 occurrences). Mechanistically, this results from the inhibition of thrombopoietin signaling during early megakaryopoiesis [3][4]. Management often requires dose interruptions, dose reductions, and the use of thrombopoietin (TPO) receptor agonists [1].
Gastrointestinal and constitutional toxicities are also highly prevalent, particularly during the first few weeks of treatment. These include nausea (up to 73%), vomiting, diarrhea, anorexia, weight loss, and fatigue/asthenia [1][4]. Hyponatremia is another common, though usually asymptomatic, adverse effect [3]. To mitigate these issues, consensus guidelines mandate prophylactic dual or triple antiemetic therapy (e.g., 5-HT3 antagonists combined with olanzapine or NK1 receptor antagonists) before initiating selinexor [1][4]. Due to these toxicities, there is ongoing debate regarding the optimal dosing schedule, with a shift from the twice-weekly 80 mg dose (used in STORM) to a better-tolerated once-weekly 100 mg dose (used in BOSTON) or even lower weekly doses in triplet combinations [1][5].
6. Future Perspectives
The future of selinexor in MM involves optimizing its use in combination regimens and expanding its role into earlier lines of therapy. Ongoing trials are evaluating selinexor alongside novel agents, such as the BCL-2 inhibitor venetoclax (particularly for patients with t(11;14) translocations) and novel cereblon E3 ubiquitin ligase modulators (CELMoDs) like mezigdomide [4].
A highly promising frontier is the integration of selinexor with T-cell-redirecting immunotherapies. Preclinical evidence suggests that selinexor can reduce T-cell exhaustion and enhance the immune susceptibility of cancer cells without compromising T-cell fitness [4]. Clinical trials are currently exploring selinexor as a bridging therapy prior to chimeric antigen receptor T-cell (CAR-T) infusion, as well as a maintenance therapy post-CAR-T to improve the durability of responses [4]. Additionally, the development of second-generation SINEs like eltanexor holds promise for delivering the robust efficacy of XPO1 inhibition with a significantly improved safety and tolerability profile [5][7].
