Commentary. Lysosomotropic agents.

Direct in situ introduction of exogenous genes into proliferating tumors could provide an effective therapeutic approach for treatment of localized tumors. Rats with a cerebral glioma were given an intratumoral stereotaxic injection of murine fibroblasts that were producing a retroviral vector in which the herpes simplex thymidine kinase (HS-tk) gene had been inserted. After 5 days during which the HS-tk retroviral vectors that were produced in situ transduced the neighboring proliferating glioma cells, the rats were treated with the anti-herpes drug ganciclovir. Gliomas in the ganciclovir- and vector-treated rats regressed completely both macroscopically and microscopically. This technique exploits what was previously considered to be a disadvantage of retroviral vectors--that is, their inability to transfer genes into nondividing cells. Instead, this feature of retroviruses is used to target gene delivery to dividing tumor cells and to spare nondividing neural tissue.

In vivo gene transfer with retroviral vector-producer cells for treatment of experimental brain tumors

The unusual regenerative properties of the liver are a logical adaptation by organisms, as the liver is the main detoxifying organ of the body and is likely to be injured by ingested toxins. The numerous cytokine- and growth-factor-mediated pathways that are involved in regulating liver regeneration are being successfully dissected using molecular and genetic approaches. So what is known about this process at present and which questions remain?

Liver regeneration: from myth to mechanism

Partial purification and characterization of hepatocyte growth factor from serum of hepatectomized rats.

Tumor cells expressing the herpes simplex virus thymidine kinase (HSV-TK) gene are sensitive to the drug ganciclovir (GCV). We demonstrate here that HSV-TK-positive cells exposed to GCV were lethal to HSV-TK-negative cells as a result of a "bystander effect." HSV-TK-negative cells were killed in vitro when the population of cultured cells contained only 10% HSV-TK-positive cells. The mechanism of this "bystander effect" on HSV-TK-negative cells appeared to be related to the process of apoptotic cell death when HSV-TK-positive cells were exposed to GCV. Flow cytometric and electron microscopic analyses suggested that apoptotic vesicles generated from the dying gene-modified cells were phagocytized by nearby, unmodified tumor cells. Prevention of apoptotic vesicle transfer prevented the bystander effect. The toxic effect of HSV-TK-positive cells on HSV-TK-negative cells was reproduced in an in vivo model. A mixed population of tumor cells consisting of HSV-TK-positive and HSV-TK-negative cells was inoculated s.c. into mice. Regression of the tumor mass occurred when the inoculum consisted of as few as 10% HSV-TK-expressing tumor cells. The bystander effect was also demonstrated in i.p. tumor studies. Initial experiments demonstrated that prolonged survival (> 70 days) occurred when a mixture containing 50% HSV-TK-positive and 50% HSV-TK-negative cells was injected i.p. followed by GCV treatment. Further, survival was prolonged for mice with a preexisting HSV-TK-negative i.p. tumor burden by injecting HSV-TK-positive cells and GCV. These results suggest that genetic modification of tumor cells may be useful for developing cancer therapies.

http://cancerres.aacrjournals.org/content/canres/53/21/5274.full.pdf

The "bystander effect": tumor regression when a fraction of the tumor mass is genetically modified.

The lack of highly exploitable biochemical differences between normal tissues and some tumors can theoretically be circumvented by a strategy utilizing gene insertion prophylactically to create tissue mosaicism for drug sensitivity, thereby ensuring that any tumor arising clonally will differ from part of the normal cell population. Elements of the strategy were tested with neoplastic BALB/c murine cell lines bearing the herpes thymidine kinase gene. Exposure to the herpes thymidine kinase-specific substrate 9-([2-hydroxy-1-(hydroxymethyl)ethoxy]methyl)guanine ablated the clonogenic potential of the cells in vitro, and administration of this drug to BALB/c mice bearing tumors produced by the cell lines uniformly induced complete regression of the tumors. The observed responses to therapy imply that the strategy may prove valuable when the genetic technology needed for its human implementation becomes available.

/pdf/tumor-chemosensitivity-conferred-by-inserted-herpes-4g67xm9x64.pdf

Tumor Chemosensitivity Conferred by Inserted Herpes Thymidine Kinase Genes: Paradigm for a Prospective Cancer Control Strategy

Carotid-to-jugular cross circulation between partially hepatectomized and normal rats, via polyethylene cannulas, stimulated incorporation of (14)C-thymidine into hepatic DNA in the normal partners when it was maintained for 19 hours at a flow rate of about 2 milliliters per minute. Cross circulation for 7 hours or less was ineffective.

https://science.sciencemag.org/content/sci/158/3798/272.full.pdf

Regeneration of Rat Liver: Transfer of Humoral Agent by Cross Circulation

Monkey-kidney cells bearing new surface antigens induced by infection with mumps virus were lysed selectively by diphtheria toxin conjugated to antibody against mumps antigens.

http://science.sciencemag.org/content/sci/169/3940/68.full.pdf

Selective Destruction of Target Cells by Diphtheria Toxin Conjugated to Antibody Directed against Antigens on the Cells

Summary The rise in hepatic DNA synthesis in response to partial hepatectomy was accelerated in rats subjected to surgery 4.5 to 8 hr or 3 days before hepatectomy. Bovine growth hormone, in doses from 0.03 to 2.0 mg, also accelerated the response to hepatectomy, but cortisone, hydrocortisone, and adrenocorticotropic hormone were ineffective. Neither surgery nor growth hormone stimulated DNA synthesis significantly in the livers of nonhepatectomized rats.

https://cancerres.aacrjournals.org/content/canres/30/9/2353.full.pdf

Accelerated Response of Hepatic DNA Synthesis to Partial Hepatectomy in Rats Pretreated with Growth Hormone or Surgical Stress

Hamster and rabbit antibodies against antigens on the surface of simian virus 40 (SV40)-transformed hamster sarcoma or lymphoma cells were purified by immunoadsorption on SV40-transformed cells and conjugated to diphtheria toxin with glutaraldehyde. Treatment with a single dose of antibody-toxin conjugate partially protected hamsters concurrently challenged with 10(3) SV40-transformed sarcoma cells, as evidenced by a reduction in tumor incidence, an increase in tumor latency, and a prolongation of the life-spans of hamsters that developed tumors. Treatment of established sarcomas was ineffective, but repeated treatment with the conjugates induced complete regressions in 20-56% of hamsters bearing established transplants in the SV40-induced lymphoma.

Antitumor Effects of Antibody-Diphtheria Toxin Coniugates. II. Immunotherapy With Coniugates Directed Against Tumor Antigens Induced by Simian Virus 40

Frederick L. Moolten

Papers

Tumor Chemosensitivity Conferred by Inserted Herpes Thymidine Kinase Genes: Paradigm for a Prospective Cancer Control Strategy

Regeneration of Rat Liver: Transfer of Humoral Agent by Cross Circulation

Selective Destruction of Target Cells by Diphtheria Toxin Conjugated to Antibody Directed against Antigens on the Cells

Accelerated Response of Hepatic DNA Synthesis to Partial Hepatectomy in Rats Pretreated with Growth Hormone or Surgical Stress

Antitumor Effects of Antibody-Diphtheria Toxin Coniugates. II. Immunotherapy With Coniugates Directed Against Tumor Antigens Induced by Simian Virus 40