PK/PD of XM22 in Children With Ewing Family of Tumors or Rhabdomyosarcoma

Inclusion Criteria: * Male or female children and adolescents aged 2 to \<18 years * Written informed consent provided by parent(s)/legal representative(s) of the pediatric patient and patient's assent if appropriate * Able to understand and/or follow study instructions alone or with parental assistance * Diagnosed with the Ewing family of tumors or Rhabdomyosarcoma * Scheduled to receive 1 of the following CTX regimens (inpatient or outpatient) * For the Ewing family of tumors: * vincristine/ifosfamide/doxorubicin/etoposide (VIDE); with concomitant sodium 2-mercaptoethane sulfonate (MESNA) according to local standards * vincristine/doxorubicin/cyclophosphamide alternating with ifosfamide/etoposide (VDC/IE); with concomitant MESNA treatment according to local standards * For rhabdomyosarcoma: * vincristine/actinomycin/cyclophosphamide (VAC) * vincristine/doxorubicin/cyclophosphamide alternating with ifosfamide/etoposide (VDC/IE); with concomitant MESNA treatment according to local standards * Chemotherapy-naïve * Body weight ≥15 kg * White blood cell (WBC) count \>2.5 x 109/L, absolute neutrophil count (ANC) ≥1.5 x 109/L, and platelet count ≥100 x 109/L (at screening and prior to CTX) * For patients aged ≥12 years, Eastern Cooperative Oncology Group (ECOG) performance status ≤2 (See Appendix A.) * Fertile patients (male or female) must use highly reliable contraceptive measures (i.e. two of the following: oral contraception, implants, injections, barrier contraception, and intrauterine device, or vasectomized/sterilized partners, or sexual abstinence). For purposes of this study, a fertile female patient is any female patient who has experienced menarche and who has not undergone tubal ligation. * Female patients who have attained menarche must have a negative urine pregnancy test at the screening visit. Exclusion Criteria: * Previous exposure to filgrastim, pegfilgrastim or lenograstim or other G-CSFs in clinical development within 6 months prior to the XM22 administration * Known hypersensitivity to filgrastim, pegfilgrastim or lenograstim or any other G-CSF in clinical development * History of congenital neutropenia or cyclic neutropenia * Any illness or condition that in the opinion of the Investigator may affect the safety of the patient or the evaluation of any study endpoint * Pregnant or nursing women * Fertile patients who do not agree to use highly reliable contraceptive measures during the entire duration of the study * Prior bone marrow or stem cell transplant, or prior radiation to ≥25% of bone marrow (e.g. whole pelvic radiation) for any reason, or any therapeutic radiation within the 3 weeks prior to the XM22 dose * Ongoing active infection or history of infectious disease within 2 weeks prior to the screening visit * Treatment with lithium at screening or planned during the study.