Inclusion Criteria:

          -  All patients and/or their parents or legally authorized representatives must sign a
             written informed consent.

          -  Patients must be > than 12 months and ≤ 21 years of age at the time of study
             enrollment.

          -  Body surface Area (For Dose Level -1): Patients must be ≥ 0.38 m² at the time of study
             enrollment.

          -  Patients with relapsed or refractory solid tumors (excluding primary central nervous
             system tumors) are eligible. Patients must have had histologic verification of
             malignancy at original diagnosis or relapse.

          -  Patients must have either measurable or evaluable disease.

          -  Patient's current disease state must be one for which there is no known curative
             therapy or therapy proven to prolong survival with an acceptable quality of life.

          -  Karnofsky ≥ 50% for patients > 16 years of age and Lansky ≥ 50 for patients ≤ 16 years
             of age.

          -  Patients must have fully recovered from the acute toxic effects of all prior
             anti-cancer chemotherapy:

               -  At least 21 days after the last dose of myelosuppressive chemotherapy (42 days if
                  prior nitrosourea).

               -  At least 14 days after the last dose of a long-acting growth factor (e.g.
                  Neulasta) or 7 days for short-acting growth factor.

               -  At least 7 days after the last dose of a biologic agent.

               -  At least 42 days after the completion of any type of immunotherapy, e.g. tumor
                  vaccines.

               -  At least 3 half-lives of the antibody after the last dose of a monoclonal
                  antibody.

               -  At least 14 days after local palliative XRT (small port); At least 150 days must
                  have elapsed if prior TBI, craniospinal XRT or if ≥ 50% radiation of pelvis; At
                  least 42 days must have elapsed if other substantial bone marrow radiation.

               -  No evidence of active graft vs. host disease and at least 84 days must have
                  elapsed after transplant or stem cell infusion.

               -  Patient must not have had prior exposure to gene vector delivery products within
                  3 months.

               -  Patients may not have had prior SGT-53. Patient who have received prior
                  topotecan, cyclophosphamide, or both are eligible.

          -  Adequate Bone Marrow Function:

               -  Peripheral absolute neutrophil count (ANC) ≥ 1000/mm³.

               -  Platelet count ≥ 100,000/mm³.

          -  Adequate Renal Function:

               -  Creatinine clearance or radioisotope GFR ≥ 70ml/min/1.73 m² OR age/gender
                  appropriate serum creatinine.

          -  Adequate Liver Function:

               -  Bilirubin (sum of conjugated + unconjugated) ≤ 1.5 x upper limit of normal (ULN)
                  for age.

               -  SGPT (ALT) ≤ 110 U/L.

               -  Serum albumin ≥ 2 g/dL.

        Exclusion Criteria:

          -  Are pregnant or breast-feeding women.

          -  Concomitant medications:

               -  Patients receiving corticosteroids who have not been on a stable or decreasing
                  dose of corticosteroid for at least 7 days prior to enrollment are not eligible.

               -  Patients who are currently receiving another investigational drug are not
                  eligible.

               -  Patients who are currently receiving other anti-cancer agents are not eligible.

               -  Patients who are receiving cyclosporine, tacrolimus or other agents to prevent
                  graft-versus-host disease post bone marrow transplant are not eligible for this
                  trial.

          -  Patients who have an uncontrolled infection are not eligible.

          -  Patients who have received a solid organ transplantation are not eligible.

          -  Patients who in the opinion of the investigator may not be able to comply with the
             safety monitoring requirements of the study are not eligible.