Recent Advances of Cell-Cycle Inhibitor Therapies for Pediatric Cancer Free

Recent preclinical and clinical studies of highly selective agents that target various regulators of the mammalian cell cycle demonstrate cell-cycle arrest, inhibition of transcription, and apoptotic cell death in models of human cancer. The cell-cycle drives proliferation of cells by the duplication of chromosomes and their distribution to a pair of genetically identical daughter cells. The fidelity of these processes relies on a series of ordered and highly regulated steps that control the transition of cells through DNA synthesis (S-phase) and cell division (M-phase), which are separated by gap phases G₁ and G₂. Activation of cyclin-dependent kinases (CDK) and association with regulatory cyclins enable successful progression through the cell cycle. Nine CDKs regulate different processes in the cell-cycle machinery, through formation of specialized and tissue-specific cyclin/CDK complexes (1). Association of cyclin D1 with CDK4 and CDK6 drives progression through G₁ stage, cyclin E and cyclin A bind CDK2 to regulate centrosome duplication and also to target helicases and polymerases during G₁–S and S-phase, and cyclin A/cyclin B/CDK1 complexes regulate the G₂–M checkpoint (1).

Inhibitory proteins further regulate activity of cyclin/CDK complexes. p27 (CDKN1B gene), p21 (CDKN1A gene), and p57 (CDKN1C gene) block the interaction of G₁-phase CDKs (CDK4 and CDK6) with their respective targets. In addition, the family of Ink4 proteins (inhibitors of kinase 4), p15 (CDKN2B gene), and p16 (CDKN2A gene) bind CDK4 and CDK6 and control mid-G₁ stage by decreasing the phosphorylation of target proteins. Over 30 cyclin/CDK/Inhibitors are implicated in cellular functions regulating transcription, repair of DNA damage, epigenetics, metabolism, proteolytic degradation, stem cell self-renewal, neuronal functions, and spermatogenesis (2).

Selective members of the CDK family of protein kinases act as oncogenic stimuli in several types of cancer (e.g., CDK1 in breast cancer and colon cancer (3, 4), CDK4 in familial melanoma (5), and CDK6 in MLL-rearranged leukemia (6). Abnormal activity is associated with the malignant transformation of cells, inhibition of DNA transcription and low response to standard drug treatment (7). Pharmacological inhibition of CDKs typically results in cell cycle arrest, apoptosis, and transcriptional repression to provide the rationale for therapeutically targeting CDKs in cancer. This review focuses on the cell-cycle inhibitors that have entered clinical trials for development against childhood cancer.

Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors have emerged as promising cell-cycle therapeutics. The CDK4 and CDK6 genes encode the CDK4 and CDK6 cyclin-dependent serine-threonine kinases (CDK 4/6), respectively. Mitogenic stimuli (e.g., estrogen and human epidermal growth factor receptor) and pro-proliferative factors (e.g., oncogenic MYC and RAS) trigger quiescent cells to express D-type cyclins and enter the cell cycle (8). These stimulate formation of CDK4/6 complexes with cyclins D1-D3, leading to phosphorylation and activation of the retinoblastoma tumor suppressor gene product (Rb). Rb protein phosphorylation releases E2F transcription factors that regulate genes that are required for G₀ or G₁ phase transition (pre-DNA synthesis) to S phase, in which DNA synthesis occurs (9). CDK4/6 signaling also has roles independent of cell-cycle regulation. These include senescence suppression via regulation of the FOXM1 transcription factor (10) and transcription in hematopoietic cells (1). High expression of D-type cyclins, genetic mutations or amplification of the CDK4 and CDK6 loci, or loss of the p16Ink4A inhibitory protein that regulates cyclin D/CDK4/6 complexes, are associated with unrestricted cancer cell growth. In addition, deletion of RB1 occurs in many tumors and accelerates proliferation independently of cyclin D–CDK4/6 activity. This suggests that activation of the cyclin D/CDK4/CDK6/Rb axis is a molecular hallmark of cancer (11). Currently, three selective CDK4/6 inhibitors are approved: palbociclib (PD-0332991), ribociclib (LEE011), and abemaciclib (LY2835219).

Palbociclib (Pfizer) is developed for treatment of ER-positive and HER2-negative breast cancer. Estrogen receptor (ER) pathway activation induces cyclin D1 and combining aromatase inhibition with CDK4/6 inhibition significantly reduces cyclin D–CDK4/6 activity (11). Palbociclib shows selective potency against CDK4 and CDK 6 (IC₅₀ 9–11 nmol/L and 15 nmol/L, respectively) in comparison with a range of other kinases (12). Preclinical studies indicate palbociclib shows antitumor activity in pediatric malignancies. Barton and colleagues (13) demonstrated that a single dose of gamma radiotherapy followed by daily treatment with palbociclib increased survival in genetically engineered brainstem glioma mouse models by 19% in comparison with radiotherapy alone. In another investigation, palbociclib plus PLX4720, an inhibitor against v-raf murine sarcoma viral oncogene homolog B1 (BRAF) extended survival in pediatric malignant astrocytoma, relative to either monotherapy (14). Response was specific for a subset of pediatric astrocytomas with BRAF (V600E) mutation and CDKN2A (encoding cyclin-dependent kinase inhibitor 2A) deficiency.

Two phase-I clinical trials are ongoing to test palcociclib in Rb-positive solid tumors and leukemia. Investigation of the maximum tolerated dose (MTD)/phase II recommended dose as well as toxicities of palcociclib in children with Rb-positive recurrent, progressive or refractory primary central nervous system (CNS) tumors is planned (NCT02255461). One clinical study for adults with various advanced solid tumors, reported dose-limiting toxicities (DLT) in patients were grade 3 neutropenia (12%), anemia (7%), and leukopenia (2%). Other common adverse events included fatigue, nausea, and diarrhea (15). For the planned pediatric clinical development, patients will receive 26 courses of palbociclib, given on days 1 to 21, over 4 weeks. Correlates of drug activity with the expression of Rb protein will determine whether this provides clinical benefit. In another planned study (NCT03132454), the safety and effects of palbociclib, when given alone and in combination with standard drugs (sorafenib, decitabine, and dexamethasone) will be investigated in patients with hematopoietic/lymphoid cancer, acute myelogenous leukemia (AML), and acute lymphoblastic leukemia (ALL). Side effects and best of different dose schedules of the standard drugs will be studied for consideration for further development.

Ribociclib (Novartis and Astex Pharmaceuticals) received recent approval for use as a frontline agent in combination with aromatase inhibition for metastatic breast cancer. Novel applications in adult patient populations are being studied for melanoma, neuroblastoma, and liposarcoma that are Rb-positive. Preclinical characterization in human neuroblastoma indicated ribociclib-induced cell-cycle arrest and reduced proliferation in 12 of 17 cell lines (mean IC₅₀ = 307 ± 68 nmol/L in sensitive lines) through selective CDK4/6 inhibition (16). Growth delay of neuroblastoma xenograft tumors and a correlation between ribociclib activity and sensitivity against MYC-amplified neuroblastoma were demonstrated. Importantly, observations of dose-dependent decreases in phosphorylated Rb and FOXM1, highlight cellular senescence could be an important mechanism associated with the clinical activity of this agent. Ribociclib was active in vitro in leukemia cells (17) and in vivo in mutant NOTCH1-driven T-ALL mouse models in combination therapy with corticosteroids and mTOR inhibitors (18). Dual inhibition of MEK1/2 (binimetinib) and CDK4/6 (ribociclib) achieved preclinical synergy (19), as well as ALK (ceritinib) and CDK4/6 (ribociclib; ref. 20) in neuroblastoma.

Ribociclib was assessed in one pediatric phase I study for 32 patients with malignant rhabdoid tumors (MRT) and neuroblastoma. Results demonstrated acceptable safety and pharmacokinetics (21). Patients received escalating once-daily oral doses (3-weeks-on/1-week-off). The MTD was 470 mg/m² and RP2D was 350 mg/m². These values were similar to those in adults. The most common grade ≥3 DLTs were fatigue (280 mg/m²; n = 1) and thrombocytopenia (470 mg/m²; n = 2). There were several adverse grade-3/4 hematologic events, including neutropenia (63%), leukopenia (38%), anemia (3%), thrombocytopenia (28%), and lymphopenia (19%) and fatigue (3%). Observations of prolonged stable disease in a subset of patients with neuroblastoma (n = 7) and primary CNS MRT (n = 2) support further clinical evaluation of combination therapies to optimize CDK4/6 targeting in neuroblastoma and MRT. An integrated analysis of ribociclib-induced cellular senescence could help to identify unresponsive tumors, and may have potentially important therapeutic implications.

Abemaciclib (Eli Lilly and Company) is an ATP-competitive, reversible kinase inhibitor with breakthrough therapy designation for patients with refractory hormone-receptor–positive (HR⁺) advanced or metastatic breast cancer. Abemaciclib shows potency against CDK4, CDK6 and CDK9 at 2, 10, and 57 nmol/L, respectively (22). One area of clinical study is the ability of abemaciclib to penetrate the blood–brain barrier (23). This agent has entered clinical evaluation for newly diagnosed diffuse intrinsic pontine glioma (DIPG) and advanced malignant brain (Grade III/IV, including DIPG; MBT) and solid tumors (neuroblastoma, Ewing sarcoma, rhabdomyosarcoma, osteosarcoma and rhabdoid tumor; NCT02644460). The primary objective of the phase I study is to identify the MTD for abemaciclib, to evaluate safety and tolerability using 2 arms, either 30–33 fractions over 6 weeks or twice daily for 28 days (which defines one cycle), each for the duration of 2 years. The secondary objectives are to assess DLTs, preliminary activity and investigate the pharmacokinetics profile starting at a dose level that is 80% of the adult dose.

Although CDK4/6 has emerged as highly relevant cell-cycle targets, agents that target other CDKs or a broad spectrum of CDKs are also under development. Flavopiridol (alvocidib) was the first pan-CDK inhibitor in human clinical trials. This drug demonstrates potent in vitro inhibition of CDK1 (40 nmol/L), CDK2 (40 nmol/L), CDK4(40 nmol/L), CDK6(40 nmol/L), and CDK7 (300 nmol/L; ref. 24). The Children's Oncology Group conducted a Phase I study to assess the safety of flavopiridol in 25 children with refractory solid tumors (25). Patients received doses of flavopiridol at 37.5 to 80 mg/m²/d over 3 consecutive days. The MTD was 62.5 mg/m²/d and the main DLTs experienced were neutropenia and diarrhea. No antitumor activity was observed in this population. The activity of flavopiridol in pediatric patients with relapsed or refractory solid tumors or lymphoma was also recently evaluated (NCT00012181) using flavopiridol IV over 1 hour on days 1 to 3 on a 21-day schedule. Results for these studies are pending.

The CDK inhibitor SCH 727965 (dinaciclib) inhibits CDKs 1, 2, 5, and 9. Preclinical evaluation of SCH 727965 by the Pediatric Preclinical Testing Program against a panel of human cell lines and pediatric-derived xenografts and ALL, induced significant delays in event free survival distribution compared with control in 64% of solid tumor xenografts and in 43% of ALL xenografts (26). The highest efficacy observed was a complete response in one leukemia model and stable disease for a single osteosarcoma xenograft. Although the use of dinaciclib in preclinical models has generated important data about targeting CDKs, the clinical development of this agent for pediatric cancer is not planned. SNS-032, an inhibitor of CDK2/9 activity, demonstrated potent single-agent toxicity in primary AML blasts and synergistic effects when combined with cytarabine (27), but showed limited clinical activity in adult patients with CLL and multiple myeloma (28).

The Aurora A, Aurora B, and Aurora C kinases are a family of highly conserved serine/threonine kinases with essential roles in mitosis and cytokinesis (29). Aurora-A localizes to chromosomes in S-phase, and expression and activation are upregulated in early M-phase by autophosphorylation at threonine 288 (Thr288). This triggers the maturation of centrosomes, formation of the mitotic spindle assembly, meiotic maturation, and cytokinesis (29, 30) to govern the correct passage of genetic material to daughter cells. Aurora A also directly phosphorylates and activates Polo-like kinase-1 (Plk-1) to promote checkpoint recovery in G₂ (31). Aurora-B is a catalytic component of the chromosomal passenger complex (CPC) that also contains the inner centromere protein (INCENP), survivin, and borealin (32). This complex governs chromosome condensation, proper chromosome–microtubule attachment at the mitotic spindle, the spindle-assembly checkpoint (SAC), and the final stages of cytokinesis (32). Aurora-C is highly expressed in testis, thyroid, and placenta and during meiosis of gametes (33). Recent studies show that increased Aurora-C levels are associated with abnormal cell division resulting in centrosome amplification and multinucleation in cells (33). NIH3T3 mouse fibroblasts overexpressing Aurora-C induced tumor formation in nude mice, to demonstrate the oncogenic potential of this isoform (34).

The observation that several tumors harbor genomic amplification of AURKA (Aurora-A kinase gene) and AURKB (Aurora-B kinase gene) supports the importance of aurora kinase signaling in cancer. This often correlates with high protein levels, and cytogenetic abnormalities including chromosome instability and aneuploidy (32, 33). Alterations in Aurora A and Aurora B vary by tumor type and reflect different subtypes. Small molecule inhibitors designed against aurora kinases target a highly conserved ATP-binding pocket, to abrogate kinase activity. These agents interrupt the mitotic checkpoint, and lead to the onset of aberrant mitosis without cytokinesis, accumulation of cells demonstrating polyploidy and cell death. First-generation agents reported off-target effects against other kinases, leading to high toxicities. Current second-generation aurora kinase inhibitors are highly specific and more potent to rapidly dividing cancer cells. 2.1 AT9283

AT9283 is a multikinase inhibitor with high potency against Aurora A and Aurora B kinase activity. AT9283 also inhibits Janus kinase 2 (JAK2) and JAK3, FMS-like tyrosine kinase 3 (FLT3), and Abelson tyrosine kinase (ABLT315I; ref. 35) with lower efficacy. Selective inhibition of Aurora-A blocks Thr228 phosphorylation, promotes formation of monopolar spindles, cell-cycle arrest at G₂–M phase and apoptosis (35). Preclinical evidence of activity of AT9283 was demonstrated by growth suppression in imatinib-resistant BCR–ABL-positive leukemic cells and in mice engrafted with BaF3/BCR-ABL, human BCR-ABL(+) cell lines, or primary patient samples expressing BCR-ABL/T315I or glutamic acid 255 to lysine imatinib-resistant mutation. Preclinical reports also indicate that AT9283 synergizes with dasatinib and enhances the repression of medulloblastoma survival and migration (36).

The first phase I/II trial with AT9283 in 33 pediatric patients with solid tumors (NCT00985868) led to a partial response in one patient diagnosed with CNS-primitive neuroectodermal tumor (PNET) and disease stabilization in 38% of patients, with manageable hematological toxicity (37). The most commonly reported drug-related toxicities were hematologic events, including neutropenia (36.4%), anemia (18.2%), and thrombocytopenia (21.2%). These were grade ≥3 in 30.3%, 6.1%, and 3% of patients. Other toxicities were fatigue, infections, febrile neutropenia and increased alanine transaminase. The phase I/II trial of AT9283 for relapsed/refractory acute leukemia (NCT01431664) did not accomplish the primary endpoint of identifying a dose for phase II assessment (38). This study was terminated because there was no evidence of clinical efficacy for the doses examined. It is likely that the evaluation of this agent in a cohort of pediatric patients with leukemia harboring the JAK, ABL, or FLT3 mutations might increase the potential of clinical responses.

Alisertib (MLN8237) inhibits Aurora-A and Aurora-B kinase with an IC₅₀ of 1.2 and 396.5 nmol/L, respectively (39, 40). Alisertib displays activity in preclinical models of lung, prostate, ovarian, and lymphoma cells (41). In the PPTP in vitro and in vivo models of childhood cancer, MLN8237 treatment led to growth repression in neuroblastoma and Ewing sarcoma cell lines as well as maintained complete responses and complete responses in neuroblastoma and ALL xenografts, respectively (39). Preclinical studies established a correlation between alisertib sensitivity and decreased AURKA copy number, in addition to a steep dose–response relationship (1 μmol/L). In AML cell lines and primary AML cells, exposure to alisertib induced antitumor activity, mediated by cell-cycle arrest and apoptotic cell death (42). Alisertib synergizes with vorinostat in vitro in pediatric leukemia, medulloblastoma, and neuroblastoma cell lines (43).

The Phase I trial to test the safety of alisertib by the Children's Oncology Group, enrolled 37 patients with advanced solid tumors. This study demonstrated that once-daily dose regimens, every 21 days were more tolerable for children than twice-daily regimens used for adults (44). DLTs were myelosuppression (3/4 patients at 100 mg/m²), mood alteration (1/6 patients at 80 mg/m²) and mucositis (1/6 patients at 45 mg/m²) on the once-daily dosing. Toxicities on the twice-daily schedule were mucositis and myelosuppression (80 mg/m²), alkaline phosphatase (1/5 patients at 60 mg/m²) and hand-foot-skin syndrome (5/11 patients). Among the 33 evaluable patients receiving alisertib in this study, a partial response (n = 1) and prolonged stable disease (n = 6) were observed. In another trial (45), single-agent alisertib once daily for 7 days of a 21-day treatment cycle administered to four pediatric patients with recurrent atypical teratoid rhabdoid tumors (ATRT), resulted in stable disease and/or durable regression for all patients. Further clinical development of alisertib focused on incorporation with mechanism-based combinations could achieve additive benefits in this patient population.

Wee1 kinase regulates the G₂–M cell-cycle checkpoint by modulating the activities of CDK1 and CDK2, to halt cell-cycle progression in response to DNA damage (46). In comparison with normal cells that repair DNA damage during G₁ arrest, deficiencies in the G₁ checkpoint in cancer cells result in DNA repair at the G₂–M checkpoint. Ataxia-telangiectasia–mutated (ATM) kinase and/or ataxia-telangiectasia-related (ATR) kinase mediate the repair of DNA double-strand breaks and DNA single-strand breaks (47), respectively. ATR also phosphorylates the checkpoint kinase CHK1, which phosphorylates Wee1. Phosphorylation of tyrosine 15 on CDK1 by Wee1 inhibits CDK1/cyclin B function, resulting in cell-cycle arrest at G₂, entry into mitosis and DNA-damage repair (46). Wee1 is also involved in stabilizing DNA replication forks and homologous recombination (HR) repair (48). Because Wee1 functionally interacts with critical regulators of mitosis, inhibition interrupts the DNA repair machinery, leading to mitotic catastrophe and apoptotic cell death (49). Wee1 is expressed at elevated levels in many adult and pediatric malignancies, including osteosarcoma (50), high-grade glioma (HGG; ref. 51), DIPG (52), and leukemia (53). Inhibition of Wee1 could sensitize cancer cells to radiation therapy (52) and chemotherapy (49), by disruption of the G₂–M checkpoint.

AZD1775 is the most frequently tested Wee1 inhibitor in preclinical and clinical studies. Combining gamma radiation and AZD1775 in HGG cell lines and orthotopic HGG and DIPG xenograft tumors led to enhanced sensitivity than achieved by radiation treatment alone (51). Kreahling and colleagues (50) demonstrated single-agent treatment of several sarcoma cell lines with AZD1775 resulted in unscheduled entry into mitosis and initiation of apoptosis. In addition, AZD1775 caused activation of cell division cycle protein 2 (CDC2; also known as CDK1) in an osteosarcoma xenograft and reduced tumor growth by 50% as monotherapy, and by approximately 70% when combined with gemcitabine (50). In hematological malignancies, AZD1775 plus panobinostat demonstrated synergistic antitumor effects in preclinical models of AML (54) and when combined with CDK inhibitor (roscovitine; ref. 53), and cytarabine (55). In the latter study, AZD1775 inhibited cytabarine-mediated S-phase arrest and prevented DNA repair. Furthermore, MYC-driven tumors or CDKN2A mutation combined with TP53 mutation could show aberrations in the G₁ checkpoint and become more reliant on the S- and G₂-phase checkpoints. S- and G₂-checkpoint abrogation caused by inhibition of Wee1 may selectively sensitize p53-deficient tumors (49). Other studies also show that AZD1775 sensitizes AML cells to cytarabine (56) and HGG cells to radiation (51) irrespective of p53 mutation status. These observations suggest that the requirement of p53 mutations for sensitivity with combination therapies using AZD1775 is context-dependent and may not be a critical consideration during development of novel therapies.

A clinical study for 25 adults with refractory solid tumors, reported DLTs in patients were supraventricular tachyarrhythmia and other common toxicities were myelosuppression and diarrhea (57). Patient recruitment for two pediatric phase-I trials to study side effects and best dose of Wee1 kinase inhibitor AZD1775 when given together with local radiation therapy in treating newly diagnosed DIPG (NCT01922076); and with irinotecan hydrochloride in treating advanced solid tumors (NCT02095132) is ongoing.

The kinesin spindle protein (KSP), also known as Hs Eg5, belongs to a family of at least 12 kinesins involved in the assembly and maintenance of the spindle during mitotic and meiotic cell division. KSP forms a homotetrameric complex that binds microtubules and mediates separation of spindle poles and assembly of the bipolar mitotic spindle (58). Inhibition of KSP leads to cell cycle arrest in mitosis with the formation of characteristic monoaster spindles (59). Monostrol, was the first small-molecule inhibitor against KSP that was discovered nearly 2 decades ago (60). Its mechanism of action was unique because all previously known small molecules that specifically affect microtubule dynamics, typically target tubulin. KSP expression is high in proliferating human tissues as well as breast, colon, lung, ovary, and uterine tumors (61).

Ispinesib (SB-715992; Cytokinetics and GlaxoSmithKline) was the first potent, highly specific small-molecule inhibitor of KSP tested in clinical trials. Ispinesib treatment against the PPTP in vivo tumor panels led to maintained complete response in one rhabdoid tumor, one Wilms tumor and one Ewing sarcoma xenograft (62). Ispinesib achieved 2 complete and 2 partial responses among 6 evaluable tumors in the ALL xenografts panel.

Reports from one phase I trial of ispinesib in 24 pediatric patients with recurrent or refractory solid tumors (NCT00363272; ref. 63) demonstrate that infusion of ispinesib for 1-hour at either 5, 7, 9, or 12 mg/m²/dose once every 3 weeks, did not achieve objective responses as monotherapy. Positive therapeutic response (n = 3), was stable disease for one patient with anaplastic astrocytoma, one with alveolar soft part sarcoma, and one with ependymoblastoma, for 4–7 courses of drug treatment.

In combination strategies in clinical trials for adults with advanced solid tumors, the best response achieved was stable disease for the evaluation of ispinesib with docetaxel (64) and with capecitabine (65). Neutropenia was the most common DLT. Second-generation inhibitors against KSP were also tested in adult trials (SB-743921, a derivative of ispinesib, MK-0731/Merck and ARRY-520/Array Biofarma). Further assessment of KSP inhibitors in prospective clinical trials for pediatric cancer are not planned.

Microtubule-targeting agents (MTA) are classed as antimitotics for cancer therapy. This class of agents has received much attention among cytotoxic agents due to their broad spectrum of clinical activity against a number of different types of cancer. The microtubule cytoskeleton governs the cell structure and motility and is involved in the intracellular transport of organelles and proteins and cell division (66). Microtubule dynamics involve the systematic reorganization of tubulin polymerization (rescue) and depolymerization (catastrophe). Stabilizing factors include microtubule-associated proteins, MAPs (MAP4, XMAP215, XMAP230/XMAP4, and XMAP310) and destabilizing factors include (Stathmin1 and XKCM1; ref. 67). Drugs that bind β-tubulin have inhibitory effects on microtubule dynamics. Consistent with the disruption of microtubules and the mitotic spindle, MTAs can arrest cell-cycle progression in mitosis, resulting in cell death (66, 68).

Clinical studies indicate activity in pediatric leukemia (69) and many solid tumors, including rhabdomyosarcoma (70). However, other tumors such as osteosarcoma are generally refractory to these drugs, either as first-line therapy or through the development of mechanisms that allow cells to undergo mitotic slippage, escape cell-cycle arrest and proliferate. These studies also reveal several limitations, such as neurological and bone marrow toxicity (69).

The taxanes (e.g., paclitaxel, docetaxel, cabazetaxel) are microtubule-stabilizing agents. These agents bind β-tubulin in a hydrophobic pocket between adjacent protofilaments, within the lumen of the microtubule (71). Synthetic compounds, for example, docetaxel, as well as analogues of existing compounds, for example, taxol-like agents, epothilones have advanced the clinical development of taxanes as a widely applied class of chemotherapeutics. Taxanes are a standard first-line treatment for metastatic breast cancer and are also used for ovarian, breast, and non–small cell lung cancer (72).

A phase I study in 31 children with refractory solid tumors showed a low overall response rate of 13% (73). Taxol was administered in 62 courses and the DLT was neurotoxicity. Positive responses were complete response for one patient, partial response for 2 patients, and a minimal response for one patient. In another study, for efficacy against refractory or relapsed leukemia (ALL), AML (n = 63), the objective response rate across all varying dose levels and schedules was <10% (69). The use of nanoparticles [e.g., nab-paclitaxel (Abraxane); nab, nanoparticle albumin bound] and antibody–drug conjugates are being explored to achieve greater specificity and therapeutic efficacy. A clinical study evaluating nab-paclitaxel combined with gemcitabine in relapsed or refractory osteosarcoma, Ewing sarcoma, rhabdomyosarcoma and other soft tissue sarcoma is underway (NCT02945800). Assessments will be response rate and progression-free survival as primary outcome measures and adverse effects as secondary outcome measures.

Pediatric phase I clinical trials of docetaxel (Taxotere) as a single-agent completed in children with refractory solid tumors (74), demonstrated positive therapeutic responses in patients who received a 1-hour infusion of docetaxel. One patient with rhabdomyosarcoma had a complete response, one with peripheral primitive neuroectodermal tumor (PPNET) had a partial response in, and there were minimal responses in two patients with PPNET. Major toxicities were neutropenia, malaise, myalgias, and anorexia. The clinical evaluation of the combination of docetaxel and gemcitabine in 28 children and adolescents with recurrent or refractory osteosarcoma, achieved 17.6% complete responses (75). Of note, the gemcitabine dose of either 675 or 900 mg/m² did not influence this response rate.

The vinca alkaloids are microtubule-destabilizing agents that bind the interface of α- and β-tubulin heterodimers and inhibit tubulin polymerization (76). Vinblastine and vincristine were the first vinca alkaloids approved for clinical use as anticancer agents. Vincristine is approved and used for induction therapy in childhood ALL (77). Several studies in pediatric patients show that vincristine has a sensitizing effect with cytotoxic chemotherapy (78–80). The liposomal formulation (sphingomyelin/cholesterol) vincristine sulfate liposome (VSL; Marqibo) in relapsed ALL was tolerated at approximately 100-fold lower doses (2.25 mg/m²)/dose of weekly VSL in 21 children with refractory solid tumors or leukemia, in comparison to standard vincristine (81). Observed clinical activity included minimal residual disease negative complete remission (n = 1) with ALL and stable disease (n = 9). Brentuximab vedotin (Adcetris), is an antibody–drug conjugate containing MMAE (monomethyl auristatin E) and an antibody targeting CD30 [tumor necrosis factor (TNF) receptor superfamily, member 8; TNFRSF8], which is expressed in classical Hodgkin lymphoma (HL) and systemic anaplastic large cell lymphoma (sALCL). CD30 is a type I transmembrane receptor and shares sequence homology in the extracellular domain with other members of the TNF receptor superfamily. Evaluation of brentuximab vedotin as frontline treatment in 16 pediatric patients with Hodgkin lymphoma (82) who received weekly dosing of 1.2 mg/kg of brentuximab vedotin, led to toxicities comparable to that of the standard-of-care backbone using vincristine. Ongoing trials for the same patient population are evaluating effects of combining brentuximab vedotin with doxorubicin (NCT01920932), dacarbazine (NCT02979522), and gemcitabine (NCT01780662).

Targeting cell regulation demonstrates efficacy in various preclinical studies in pediatric malignancies. Validation of effect in clinical studies is ongoing (Fig. 1, Table 1). However, a single-agent efficacy signal is not likely in studies focused on small patient subgroups (e.g., metastatic and recurrent disease or patients with specific mutations; ref. 36). The biologic rationale for the development of agents targeting cell-cycle regulation is sound and early efficacy signals reinforce the need for combination studies in patients at highest risk for adverse outcomes.

Early phase studies of cell-cycle regulators for pharmacological inhibition in the treatment of cancer in children demonstrate appreciable tolerability, and support the incorporation into combination approaches. These studies also demonstrate differences in DLTs in targeting different cell-cycle components. One primary concern with the development of therapeutic interventions of the cell-cycle pathway is that strategies will not distinguish between normal dividing cells and cancer cells. At present, HR-positive status in breast cancer tumors is the only clinical indicator for these agents (83). Because mitogens regulate D-type cyclins, understanding the connection between mitogenic signals and the cell cycle will help identify potential predictive biomarkers. Validation and integration of functional assays will also be required to successfully translate these inhibitors for pediatric use.

In addition to agents discussed in this review (Fig. 1, Table 1), the development of inhibitors against other components of the cell-cycle machinery, have not yet been explored in pediatric patients. For example, investigations of antitumor activity for inhibitors against ATM (KU-55933, KU-59403; KuDOS Pharmaceuticals, AstraZeneca) and ATR (Schisandrin B, AZD6738) have advanced to early-phase trials in adult patients with cancer. Simultaneous targeting of proteins that cross-talk (e.g., ATM/ATR and Wee1 at the G₂–M checkpoint) could show results that are more robust that targeting a single checkpoint protein. In addition, combining aurora kinase or Wee1 kinase inhibition with disruption of microtubule dynamics might achieve higher sensitivity of cancer cells. The design of these combination strategies will rely on the outcome of trials that are completed or underway. Combination clinical studies with immunotherapy are ongoing in adults. Of note, the development of other novel agents that indirectly affect cell division are ongoing. The anti-tropomyosin drugs represent a novel class of anti-actin agents that target tropomyosins that are important for tubulin stability in the kinetochore and are being studied in combination with antimitotics (84).

Several other common factors may influence clinical trial outcome for pediatric patients. Some studies are withdrawn when trial doses are ineffective. Pediatric tumors are rare and biologically distinct from tumors in adults, and developmental changes in infancy and adolescence associated with growth and maturation of organs, as well as alterations in metabolism may alter drug effects in pediatric patients. It is likely the extrapolation of efficacy from adult and other data to the pediatric population by using relatively low starting doses in children, in comparison with the adult MTD, could diminish clinical efficacy for eligible patients. The extrapolation of preclinical results to the clinical setting requires more focused studies on doses and dose schedules in relevant preclinical animal models to successfully correlate the systemic exposure of agents tolerated in humans compared with mice. A greater understanding of the development and progression of pediatric disease is crucial for optimizing study designs for control groups, stratification, and biomarker assay development. The observation that activity of an agent in pediatric leukemia may be limited to patients with specific mutations (38), or Rb status in solid tumors (16), would require recruitment of select patient populations.

Finally, studies that are closed prematurely when adult trials do not reach their primary endpoint can limit availability of new drugs before pediatric studies are complete. This was seen when MLN8237 (alisertib) failed to reach its primary endpoint for adult lymphomas and lung cancers during early-phase studies in pediatric patients. Despite promising results for pediatric use, drug development was halted. Table 2 lists the current regulatory status and indications of the agents discussed in this review. This highlights the vulnerability of pediatric drug development to outcomes in adult trials. Current clinical studies to determine the most efficacious and safe approaches are critical for the advancement of drugs for pediatric use and provide valuable information and new hope to improve the quality of life to children with these rare diseases.

No potential conflicts of interest were disclosed.

This work is supported by the Clinical and Translational Research (CTR) Pilot Award to V.B. Sampson under grant number NIHU54-GM104941.