This is a Phase I, open label study aimed at assessing the pharmacokinetics, pharmacodynamics, the efficacy, safety, and tolerability of a single injection of XM22 in children with Ewing family of tumors or rhabdomyosarcoma scheduled to receive chemotherapy (CTX)

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.