A Potential Therapeutic Strategy to Combat Leukemia Virus Infection

Abstract

To test the concept that a replication-competent retrovirus carrying a suicide gene could have potential utility in the control of the natural virus infection in mammalian species, we constructed derivatives of a feline leukemia virus (FeLV) that is commonly associated with leukemia-lymphomas in this species. The FeLV, Rickard strain, subgroup A (FRA) genome contained at the 3' end of the envgene, an insert of an internal ribosomal entry site (IRES) linked to cDNA sequence of either herpes simplex virus thymidine kinase (HSV-TK) or a truncated HSV-TK (HSV-ATK) or yeast cytosine deaminase (CD). These constructs were transfected into feline fibroblast cells (H927). The viruses produced were determined to be replication-competent. The stable propagation of the full-length transgene was, however, dependent on the size of the insert, IRES-CD being the smallest in size (1031 bp) exhibiting maximal stability for at least up to six months. The protein products of the transgenes could be detected, despite the appearance of deleted proviruses at late passages. The transduced cells were susceptible to cytotoxic killing when the appropriate prodrug, ganciclovir (GCV), acyclovir (ACV) or 5-fluorocytosine (5-FC) was added to the culture medium. H927 cells, infected with another subgroup of FeLV, namely, FeLV-B or FeLV-C, could be superinfected by the FRA-suicide gene viruses and thus, subjected to killing. Interestingly, at an early stage of infection by the parental FRA, H927 cells could also be reinfected by the same subgroup FRA constructs to induce the suicide effect. Among the three constructs, the vector with the CD gene was determined to be superior to others in terms of stability, therapeutic index and bystander effect in the cell culture test system. While the in vivo correlates of the therapeutic effect in the feline model remain to be determined, our results do encourage investigation of the same concept in the control of HTLV and, perhaps even, HIV infection in humans.

Design and Selection of Toca 511 for Clinical Use: Modified Retroviral Replicating Vector With Improved Stability and Gene Expression

“…In the context of incomplete intratumoral viral spread, efficacy may be more dependent on the rate of 5-FC conversion to 5-FU in infected tumor cells, and the 5-FU diffusing to, and killing, nearby noninfected dividing tumor cells-the so-called "bystander effect". 37 It is unclear whether, even if there were demonstrably better individual cell killing with the CD-UPRT and CD-OPRT hybrid genes, this would be helpful therapeutically. 35 The planned therapeutic strategy relies on efficiently infecting tumors, allowing the virus to replicate, dosing with 5-FC to kill dividing infected tumor cells, then allowing a "rest" period where the virus can replicate and infect any residual dividing, and as yet uninfected tumor cells.…”

Lyophilization Does Not Inactivate Infectious Retrovirus in Systemically Infected Bone and Tendon Allografts

“…11,33,34,52 FeLV, a retrovirus with a structure and replication cycle similar to that of HIV, is widely used as an animal model for HIV. 13,16,23,27,31,32,36,38 Previous studies have demonstrated that a variety of musculoskeletal tissues, including ligament and bone, transmit the FeLV retrovirus after allotransplantation. 11,33,34,52 In addition, we have shown that conventional preservation techniques including deep freezing and double freeze-thaw cycles do not prevent retroviral transmission.…”

Fratricidal Retroviruses: A New Twist in Gene Therapy

“…16,17 In their paper in this issue of Cancer Biology & Therapy, Pan et al report that recombinant FeLV, Rickard strain sub group A (FRA), containing the conditionally cytotoxic genes, HSV thymidine kinase (HSV-TK) gene or the cytosine deaminase (CD) gene from yeast, confers cytotoxicity to infected cells following treatment with the appropriate prodrug enzyme substrates. 18 A significant bystander effect leading to growth inhibition of uninfected cells by co-culture with the FRA-suicide gene vector-infected cells was also observed. Pan et al next asked whether the FRA-suicide viruses would be cytotoxic for cells already infected with wild-type FeLV.…”