Rhabdomyosarcoma (RMS) is a malignant tumor of skeletal myoblast-like cells and is the most common pediatric soft-tissue sarcoma, with an incidence of approximately 4.5 cases per million children/adolescents per year (1, 2). Although 5-year overall survival exceeds 70%, approximately one-third of patients experience progression/relapse within 13 months of diagnosis, and 5-year post-relapse survival remains approximately 24% (3, 4). Treatment for relapsed disease typically consists of multi-agent chemotherapy combinations with or without surgery and radiotherapy and often results in minimal benefit, indicating an urgent need for novel therapeutics, especially for patients with multiple relapses (3).

Two major RMS subtypes exist—PAX3/7-FOXO1 fusion-negative, also known as embryonal RMS, and PAX3/7-FOXO1 fusion-positive, also known as alveolar RMS (ARMS), each with a unique array of molecular alterations (5). The PAX3/7-FOXO1 fusion-negative subtype commonly exhibits loss of heterozygosity at the 11p15 locus, which contains the IGF-II gene and often has FGFR1 and NRAS mutations (6). In contrast, the PAX3/7-FOXO1 fusion-positive subtype, the most aggressive RMS subtype, is characterized by a translocation between the PAX3/7 and FOXO1 genes and amplification of MYCN and cyclin-dependent kinase (CDK) 4 (7, 8). Moreover, CDK4 is amplified in approximately 25% of PAX3-FOXO1 cases and approximately 4% of PAX7-FOXO1 cases (9). Other aberrations in the CDK4/6 pathway have been reported, including the homozygous deletion of CDKN2A/B and amplification of CCND2 and CCND3 (10–12). CDK4/6 pathway alterations may render fusion-positive cells vulnerable to targeted treatment with a CDK4/6 inhibitor.

Abemaciclib is an oral, potent, and selective CDK4/6 inhibitor approved for certain types of early stage and metastatic breast cancers (13–15). Abemaciclib is a unique CDK4/6 inhibitor due to its continuous dosing schedule, greater relative potency to CDK4 than CDK6, and its ability to penetrate the central nervous system (16–19). Abemaciclib blocks CDK4/6 from phosphorylating the retinoblastoma tumor suppressor, which results in G1 arrest and inhibition of cancer cell growth (16). The phase 1b study, I3Y-MC-JPCS (JPCS), evaluated abemaciclib in combination with temozolomide and irinotecan in pediatric and young adult patients with relapsed/refractory solid tumors. This case report examines the notable results of an adolescent patient enrolled in the JPCS study with relapsed ARMS who achieved a durable complete response.

An 8-year-old White male was diagnosed with stage IV PAX3-FOXO1 fusion-positive ARMS of the right foot and multiple lymph nodes with confirmed disease ( Figure 1 ). Tumor biomarker analysis following the fourth relapse revealed a MYCN mutation (c.131C>T resulting in p.Pro44Leu amino acid alteration) and amplification of CDK4, MYCN, ERBB3, GLI1, and FGFR4. No RB1 alterations were detected. The patient had received five treatment regimens over approximately 7 years ( Table 1 ). During this time, the patient underwent multiple surgeries and four courses of radiotherapy targeting lesions in the lower extremity, mesenteric nodes, muscle, and abdominal-pelvic regions.

After the fifth relapse, the patient enrolled in JPCS, a phase 1b trial with a 3 + 3 dose-escalation design, for pediatric and young adult patients with relapsed/refractory solid tumors. The patient was assigned to abemaciclib (55 mg/m² twice daily [BID continuously]) in combination with both temozolomide (100 mg/m² daily) and irinotecan (50 mg/m² daily) on days 1 to 5 of 21-day cycles. No preexisting conditions were reported. The disease sites at study entry included five target tumors (located in the supraclavicular lymph node, mediastinal lymph node ×2, and iliac lymph node ×2) and four non-target tumors (located in the supraclavicular lymph node, mediastinal lymph node, iliac lymph node, and retroperitoneum; Figures 2a–c ). The patient received 35 cycles of abemaciclib, the first 12 in combination with temozolomide and irinotecan (discontinuation of chemotherapy was permitted after cycle 12 per protocol). Treatment compliance ranged from 98% to 100% for all three study drugs.

Computed tomography scans were obtained after cycle 2, cycle 4, and every third cycle thereafter. Partial response was achieved at the first scan ( Figures 2d–f ), followed by complete response at day 78. The last disease-free scan is shown in Figures 2g–i . The patient had ongoing benefit for 22.6 months until disease progression when five new tumors (left hilar adenopathy, lumbar and sacral spine, L5 vertebra, D8 vertebra, and femurs) were identified ( Figure 3 ). Progression-free survival was 23.7 months.

The patient tolerated study treatment well and experienced no dose-limiting toxicities or serious adverse events. The most frequent treatment-related adverse events were thrombocytopenia, diarrhea, dizziness, nausea, vomiting (all with a maximum severity of grade 1), and neutropenia (maximum severity of grade 3). Abemaciclib was withheld for 3 days due to an adverse event of grade 3 aspartate aminotransferase increase that was deemed unrelated to treatment, and there were no dose reductions. There were no dose modifications of temozolomide or irinotecan.

Pharmacokinetics assessments demonstrated that abemaciclib exposure and that of its M2 and M20 metabolites as well as temozolomide, irinotecan, and its metabolite SN-38 were consistent with that in other patients who received the same dose (data on file (20);).

ARMS is a recalcitrant disease with rapid spreading, dismal therapeutic response, and high relapse rates (21). Given the poor prognosis, new targeted therapeutic approaches are needed. This case report highlights the notable results in a patient with relapsed ARMS who was treated with temozolomide and irinotecan in combination with the CDK4/6 inhibitor, abemaciclib.

Deregulation of the CDK4/6 pathway has been reported in ARMS, although its contribution to oncogenesis and disease progression is not fully understood (21). The primary CDK4/6 pathway alteration in ARMS is CDK4 amplification, which is present in about 25% of PAX3–FOXO1-positive tumors and approximately 4% of PAX7–FOXO1-positive tumors (9). The mechanism of CDK4 amplification and overexpression is likely via amplification of the 12q13-q14 amplicon (21, 22). Based on the frequent CDK4 alterations, abemaciclib may serve as a rational therapeutic approach to counteract the resulting cell cycle abnormalities and inhibit ARMS progression.

The case presented herein demonstrates a prolonged response to treatment with abemaciclib and standard chemotherapy in a patient with fifth relapse, stage IV, CDK4-amplified, PAX3-FOXO1 fusion-positive ARMS. The patient discontinued chemotherapy after 12 cycles and continued abemaciclib monotherapy for 23 additional cycles, which suggests that abemaciclib significantly contributed to the sustained complete response.

In addition to CDK4 amplification, this patient’s tumor also harbored FGFR4 amplification, which has been associated with aggressive RMS behavior and therapeutic resistance (23). While the contribution of FGFR4 to the observed response is unclear, its presence highlights the complexity of molecular alterations in ARMS and the potential need to consider co-alterations in future studies (24).

Recent studies in other CDK4-amplified sarcomas, such as dedifferentiated liposarcoma and synovial sarcoma, have demonstrated synergistic antitumor effects and prognostic significance of CDK4/6 inhibition (25, 26). These findings further support the broader relevance of investigating CDK4/6 across soft tissue sarcomas and further support exploration of this therapeutic strategy in ARMS.

The findings in this case contrast with the results of studies of other CDK4/6 inhibitors as monotherapy or in combination, which were less promising in a limited number of patients with ARMS-containing alterations in the CDK4/6 pathway (27, 28). Likewise, two other patients with RMS (unknown subtype) in the JPCS Part A study did not achieve durable responses. Thus, the molecular mechanisms rendering sensitivity in some cases and resistance in other cases remain elusive.

Preclinical studies have postulated mechanisms to explain CDK4/6 inhibitor response. A study evaluating fusion-positive RMS suggested that CDK4/6 inhibitor sensitivity depends on the level of CDK4 amplification; specifically, all fusion-positive models evaluated exhibited sensitivity, but models with lower CDK4 amplification (though higher than no amplification) were more sensitive than those with higher CDK4 levels (9). The researchers hypothesized that cyclin D1 expression may saturate CDK4 and limit the effects of CDK4 overexpression (9). Furthermore, preclinical evidence has linked Rb1 loss to ARMS progression and demonstrated that downregulation of pRb expression leads to CDK4/6 inhibitor resistance (29). Thus, effective CDK4/6 inhibition likely hinges not only on appropriate CDK4 levels but also on enduring Rb functionality.

This case report is inherently limited by its single-patient nature which restricts the generalizability of the findings. While the prolonged complete response is notable, broader conclusions regarding the efficacy of CDK4/6 inhibition in ARMS require validation through larger controlled studies.

The evidence presented in this case report strengthens the argument that CDK4/6 may play an important role in the pathogenesis of CDK4-amplified ARMS and that abemaciclib in combination with temozolomide and irinotecan may abrogate tumor growth and improve outcomes. However, this evaluation is limited to a single patient, and we do not yet fully understand the biological or molecular basis for this patient’s exceptional initial response or the factors that led to eventual disease progression. It should be noted that this patient had achieved complete responses when treated with prior regimens and thus may have a unique molecular profile yielding favorable outcomes. Nonetheless, a nearly 2-year treatment response for fifth relapse ARMS is remarkable. These results provide the first insights into the clinical activity of abemaciclib, temozolomide, and irinotecan in CDK4-amplified ARMS, and additional studies may be warranted to further understand the role of CDK4/6 alterations in ARMS. Further research could explore biomarker-guided trials to identify benefit of CDK4/6 inhibition and evaluate this combination in prospective studies across molecularly defined RMS subtypes. Such studies could clarify the mechanisms of response and resistance and inform patient selection strategies.

Novel treatments that exploit targetable alterations are needed to improve outcomes for relapsed ARMS. This case report describes an adolescent male with fifth relapse, CDK4-amplified, PAX3-FOXO1 fusion-positive ARMS who had a remarkable response to treatment with the CDK4/6 inhibitor abemaciclib in combination with temozolomide and irinotecan. These results underscore the importance of exploring CDK4/6 inhibition as a potential treatment strategy for ARMS.