Cyclophosphamide facilitates antitumor efficacy against subcutaneous tumors following intravenous delivery of reovirus.
OBJECTIVE
The purpose of the present study was to investigate whether it is possible to achieve truly systemic delivery of oncolytic reovirus, in immunocompetent hosts, using cyclophosphamide to overcome some of the barriers to effective intratumoral delivery and replication of i.v. injected virus.


METHODS
I.v. delivery of reovirus was combined with different regimens of i.p. administered cyclophosphamide in C57Bl/6 mice bearing established s.c. B16 tumors. Intratumoral viral replication, tumor size, and survival were measured along with levels of neutralizing antibody (NAb) in the blood. Finally, differential toxicities of the virus/cyclophosphamide regimens were monitored through viral replication in systemic organs, survival, and cardiac damage.


RESULTS
Repeated i.v. injection of reovirus was poorly effective at seeding intratumoral viral replication/oncolysis. However, by combining i.v. virus with cyclophosphamide, viral titers of between 10(7) and 10(8) plaque-forming units per milligram were recovered from regressing tumors. Doses of cyclophosphamide that ablated NAb were associated with severe toxicities, characterized by viral replication in systemic organs--toxicities that are mirrored by repeated reovirus injections into B-cell knockout mice. Next, we restructured the dosing of cyclophosphamide and i.v. virus such that a dose of 3 mg cyclophosphamide was administered 24 h before reovirus injection, and this schedule was repeated every 6 days. Using this protocol, high levels of intratumoral viral access and replication ( approximately 10(7) plaque-forming units per milligram tumor) were maintained along with systemically protective levels of NAb and only very mild, non-life-threatening toxicity.


CONCLUSIONS
NAb to oncolytic viruses play a dual role in the context of systemic viral delivery; on one hand, they hinder repeated administration of virus but on the other, they provide an important safety mechanism by which virus released from vigorous intratumoral replication is neutralized before it can disseminate and cause toxicity. These data support the use of cyclophosphamide to modulate, but not ablate, patient NAb, in development of carefully controlled clinical trials of the systemic administration of oncolytic viruses.
Qiao
Jian
J
Molecular Medicine Program, Mayo Clinic College of Medicine, Rochester, Minnesota 55902, USA.
Wang
Hongxun
H
Kottke
Timothy
T
White
Christine
C
Twigger
Katie
K
Diaz
Rosa Maria
RM
Thompson
Jill
J
Selby
Peter
P
de Bono
Johann
J
Melcher
Alan
A
Pandha
Hardev
H
Coffey
Matt
M
Vile
Richard
R
Harrington
Kevin
K
D000818:Animals; D000914:Antibodies, Viral / Q000097:blood / Q000276:immunology; D018906:Antineoplastic Agents, Alkylating / Q000008:administration & dosage* / Q000009:adverse effects; D003131:Combined Modality Therapy; D003520:Cyclophosphamide / Q000008:administration & dosage* / Q000009:adverse effects; D008546:Melanoma, Experimental / Q000628:therapy*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D050130:Oncolytic Virotherapy / Q000009:adverse effects / Q000379:methods*; D012087:Reoviridae / Q000502:physiology*
D000914:Antibodies, Viral; D018906:Antineoplastic Agents, Alkylating; D003520:Cyclophosphamide