¹Department of Pharmacology and Experimental Therapeutics, University of Maryland, School of Medicine, Baltimore, MD, USA

Correspondence: Dr L Aurelian, Department of Pharmacology and Experimental Therapeutics, University of Maryland, School of Medicine, 655 West Baltimore Street, 4-002, Baltimore, MD 21201-1559, USA. E-mail: laurelia@umaryland.edu

Received 19 February 2009; Revised 17 May 2998; Accepted 11 June 2009; Published online 1 October 2009.

Abstract

Malignant melanoma is a highly aggressive and drug-resistant cancer. Virotherapy is a novel therapeutic strategy based on cancer cell lysis through selective virus replication. However, its clinical efficacy is modest, apparently related to poor virus replication within the tumors. We report that the growth compromised herpes simplex virus type 2 (HSV-2) mutant, PK, has strong oncolytic activity for melanoma largely caused by a mechanism other than replication-induced cell lysis. The ratio of dead cells (determined by trypan blue or ethidium homodimer staining) to cells that stain with antibody to the major capsid protein VP5 (indicative of productive infection) was 1.8–4.1 for different melanoma cultures at 24–72 h post-infection. Cell death was due to activation of calpain as well as caspases-7 and -3 and it was abolished by the combination of calpain (PD150606) and pancaspase (benzyloxycarbonyl-Val-Ala-Asp-fluormethyl ketone, z-VAD-fmk) inhibitors. Upregulation of the autopahgy protein Beclin-1 and the pro-apoptotic protein H11/HspB8 accompanied PK-induced melanoma oncolysis. Intratumoral PK injection (10⁶–10⁷ plaque-forming unit (pfu)) significantly reduced melanoma tumor burden associated with calpain and caspases-7 and -3 activation, Beclin-1 and H11/HspB8 upregulation and activation of caspase-1-related inflammation. Complete remission was seen for 87.5% of the LM melanoma xenografts at 5 months after treatment termination. The data indicate that PK is a promising virotherapy for melanoma that functions through virus-induced programmed cell death pathways.