TXT is a non-cytotoxic precursor derived from the needles of Taxus specimens that has been chemically modified to yield a taxol-type antitumor agent that is threefold higher in intracellular concentration than paclitaxel. TXT also exhibits long residence time in the cell (42). It acts mainly through the promotion of stable microtubule polymerization and the inhibition of depolymerization, thereby significantly reducing the number of free microtubules, inhibiting spindle separation at the two poles, blocking the G and M phases and ultimately inhibiting tumor cell mitosis and proliferation (47).

Certain scholars have reported that TXT can inhibit the proliferation of OS cells and induce apoptosis, and that such inhibition is time- and dose-dependent (48,49). Combination therapy using TXT and gemcitabine (GEM) has also been reported to enhance antitumor effects (50,51). A number of scholars (52) have used chemotherapy-related drug target gene detection via the application of second-generation high-throughput sequencing technology in patients with OS undergoing surgical resection. The results of such studies indicate that TXT induces OS cell apoptosis, and that OS cells can achieve moderate TXT sensitivity (53.3%).

Palmerini et al (53) used TXT chemotherapy for 14 cases of recurrent OS, 1 case of partial remission, and 1 case involving a patient in stable condition. Zwerdling et al (54) used single-drug TXT chemotherapy, reporting 1-year and 5-year overall survival rates of 24 and 6%, respectively. These observations indicate that such approaches may be useful in the treatment of metastatic and relapsed/refractory OS, and OS that is unresponsive to first-line treatment.

GEM is a fluorinated analog of the nucleoside deoxycytidylic acid, which is converted into active diphosphate and nucleoside triphosphates by the action of nucleoside kinases in the cell. Diphosphate inhibits ribonucleotide reductase and is involved in deoxycytidine triphosphate competition as a nucleoside triphosphate. Ribonucleotide reductase inhibition can activate self-generating mechanisms, thereby increasing the entry of nucleotides into cells. DNA synthesis terminates when the GEM triphosphate metabolite enters the DNA to produce additional nucleotides. This mechanism can develop resistance to DNA repair enzymes, which may represent a mechanism for overcoming drug resistance (55). The results of a small-sample phase II clinical trial have indicated that the effective rate of GEM monotherapy is <10%. Recent studies (47,53,56,57) have also indicated that TXT can increase the expression of thymidine phosphorylase in tumor tissue, thereby enhancing the antitumor activity of GEM, resulting in synergistic antitumor effects ranging from 17–30% (58). He et al (51) reported that 80% of patients [median OS duration: 13 months; median progression-free survival (PFS): 6–7 months] receiving GEM in combination with TXT as second-line therapy experienced 12- and 24-month survival rates of 66 and 80%, respectively.

Qi et al (48) treated 18 patients with relapsed/refractory OS using TXT combined with GEM, reporting a tumor control rate of the rate [(CR+PR+ stable disease (SD)] of 22.3% and a median overall survival of 8 months. Furthermore, Mora et al (59) used the same chemotherapy protocol, reporting a remission rate (CR+PR) of 50% and a median remission duration of 10 months. O'Day and Gorlick (60) reported that the overall response rate of patients with OS was 43% following combined TXT+GEM chemotherapy. Song et al (55) also reported a response rate (CR+PR) of 23.5% and a control rate (CR+PR+SD) of 41.2%, with a 1-year overall survival rate of 53.6% and a median remission duration of 11.2 months in patients receiving TXT+GEM for recurrent/refractory OS. Certain scholars have also applied this strategy for the treatment of patients undergoing limb-salvage surgery, reporting remission rates of 13.0% and a median overall survival period of 9 months (51). Additional studies have added arsenic trioxide (As₂O₃) to the TXT+GEM combination regimen for the treatment of first-line chemotherapeutic drug resistance in patients with OS lung metastasis, reporting an overall effective rate (CR+PR) of 34.6%, along with 1-,2-, and 4-year survival rates of 61.5, 38.4 and 15.4%, respectively (55). Additional studies have also reported disease control rates between 28.5–41.2% (55,58) and median overall survival times of 7–11 months following TXT+GEM treatment in patients with refractory/metastatic OS (53). Therefore, the TXT+GEM regimen, for which a relatively larger amount of evidence has been reported relative to other recommended regimens, was the first to be recommended (51) (as shown in Table II).

Sorafenib acts to inhibit a variety of kinases present in the cell and at the cell surface, including RAF kinase, vascular endothelial growth factor receptor-2, vascular endothelial growth factor receptor-3, platelet derived growth factor receptor-β, KIT Proto-Oncogene Receptor Tyrosine Kinase (KIT) and Fms Related Tyrosine Kinase 3 (FLT-3). A recent phase II study revealed that the median PFS was 4 months for sorafenib-treated patients with first-line failure and unresectable OS, with a clinical benefit rate of 29 and a 17% increase in clinical benefit after 6 months (61). Such observations provide the first evidence for the efficacy of small-molecule agents targeted toward the second-line treatment of lung metastasis in OS.

Everolimus is a derivative of sirolimus (also known as rapamycin), which is a classical inhibitor of the mechanistic target of rapamycin target and a novel macrolide immunosuppressive agent. Rapamycin prevents T lymphocytes and other cells from transitioning from the G₁ to S phases via blockage of signaling pathways involving various cytokines, which serve an important role in immunosuppression. Grignani et al (61,62) speculate that sorafenib and combined therapy using sorafenib and everolimus are promising for the treatment of patients with advanced or unresectable OS, although they did not attain the prespecified target of 50% or greater PFS within 6 months.

CTX is a bifunctional alkylating and cell cycle nonspecific agent that interferes with the DNA and RNA function, exerting its most apparent effects during the S phase via DNA cross-linking and inhibition of DNA synthesis. CTX first transforms into aldehydes and phosphorus amide in the liver in vitro and without antitumor activity. However, aldehyde amide in tumor cells is transformed into amide nitrogen mustard and acrolein, which exert cytotoxic effects.

Etoposide is a cell cycle-specific antitumor drug that acts on DNA topoisomerase II to form the reversible complex of drug-enzyme-DNA stabilization, thereby hindering DNA repair. This complex can be reversed by cessation of drug treatment, so that the damaged DNA undergoes repair and cytotoxicity is reduced.

A phase II study reported that combined treatment using CTX and etoposide X2 arrested OS progression in a significant number of patients (54%), with a relatively good tolerability and self-limiting toxicity that was resolved in all cases (11).

TPT is an inhibitor of topoisomerase I, which binds to the topoisomerase I-DNA complex and prevents reconnection of these single strand breaks. TPT is an S-phase specific drug whose cytotoxicity influences the synthesis of DNA. Saylors et al (12) reported that the combination of CTX and TPT is effective in the treatment of rhabdomyosarcoma, neuroblastoma and Ewing's sarcoma. Stabilization of disease was also observed in patients with OS, although objective responses were rare. Furthermore, TPT therapy can be initiated within the range of acceptable hematopoietic toxicity with the use of filgrastim support.

IFO is a cell-cycle nonspecific drug that is mainly activated by hydrolysis of the first four carbons, following which 4-hydroxy-IFO automatically forms aldehyde IFO, which decomposes into phosphorus amide nitrogen mustard and acrolein. The cytotoxic effect of IFO influences cross-linking with DNA. The cell cycle increases in proportion to G₂+M, and the G₂ phase is delayed following administration of IFO. Magnan et al (63) conducted a retrospective chart review and reported that high doses of IFO resulted in a 4-year event-free survival (EFS) rate of 27% and an overall survival rate of 39%.

The combination of etoposide and high-dose IFO is effective for induction of chemotherapy in patients with metastatic OS (3). Le Deley et al (17) reported a 5-year EFS rate of 62% for the entire study population, which was slightly higher in the etoposide-IFO arm than in the doxorubicin arm. Furthermore, the 5-year overall survival rate of the entire population was 76%. In total, ~43% of patients in the etoposide-IFO arm were event-free at 3 years without having received any doxorubicin or DDP, thus avoiding the risk of long-term cardio- and cardiotoxicity.

Pemetrexed is a multi-target antifolate agent that significantly inhibits thymidylate synthase, dihydrofolate reductase and glycine ribonucleoside formyl transferase to interfere with folic acid metabolic pathways. Studies have revealed that such treatment may be effective as second-line chemotherapy in patients with MTX-resistant OS (45). Endostar has been combined with pemetrexed for the treatment of recurrent, drug-resistant OS and pulmonary metastases of OS. Furthermore, recent observations indicate that this treatment is effective and results in significant improvements in the quality of life without significant adverse reactions (64–67). Although trial results regarding second-line chemotherapy with pemetrexed for patients with OS lung metastases have been positive, further evidence regarding monotherapy and combination therapy using this agent is required.

Liposome-encapsulated muramyl tripeptide-phosphatidyl ethanolamine (L-MTP-PE). L-MTP-PE is an analog of muramyl dipeptide with the ability to activate macrophages and monocytes. Phase III clinical trials have indicated that L-MTP-PE can enhance immune system function (68). Mori et al (69) reported that infusion of L-MTP-PE can reduce the recurrence of lung metastases following surgery, and that the survival rate was higher than that of patients in the concurrent chemotherapy group. In recent years, L-MTP-PE has gained attention for its effects on pulmonary metastasis of OS and may prove effective as a second-line chemotherapy agent.

Samarium-153 ethylenediaminetetramethylene phosphonate. The ¹⁵³Sm nuclide is ideal for the diagnosis and treatment in nuclear medicine and is mainly used in the preparation of therapeutic radiopharmaceuticals, including ¹⁵³Sm-EDTMP (ethylene diamine tetramethylene phosphonic acid ethylene diamine tetramethylene phosphonic acid), which has a high affinity for pro-tumor characteristics and bone. This agent has been used to treat bone metastases and relieve bone pain, although it can also be used for the treatment of primary bone cancer. Furthermore, bone-specific irradiation can be induced using ¹⁵³Sm-EDTMP with peripheral-blood progenitor-cell support (70).

Radium 223 chloride, an α-particle radioactive material, was the first particle to be used in radiation therapy drugs. The use of this agent is advantageous in that 95% of the decay can be released in the form of α-particles, resulting in less damage to normal cells. Radium 223 chloride simulates the main constituent of bone calcium, although the actual course of treatment requires 6 injections/month. Furthermore, the drug is transferred to sites with metabolically active cells and metastatic bone cancer cells following injection. α-particles are then released, resulting in the inhibition of bone metastases and cancer cell proliferation. Radium-223 is associated with low marrow toxicity and increased radiobiological effectiveness, particularly in bone-forming types of cancer, when compared with samarium-153-EDTMP (18,19). Radium 223 may have greater potential for widespread use against bone metastases in OS since it may achieve safe and effective decrease of tumor burden and facilitate improvements in surgery and/or radiotherapy in patients with unresectable, large or metastatic tumors.

Multiple molecular-based therapies have also been developed for the treatment of OS, including anti-C-X-C chemokine receptor type 4 drugs (71), matrix metalloproteinase inhibitors, selective cyclooxygenase-2 inhibitors (72), type 1 insulin like growth factor receptor, and osteoclast-targeted therapy (73) The main factors affecting the function of osteoclasts are receptor activator NF-κβ ligands, osteoprotective proteins, bisphosphonates and Src inhibitors (74) However, targeted therapies also produce resistance and adverse effects (Table III), thus necessitating further investigation (61,75–78).