Lack of Proper Response Indicators

An equally important question that remains largely unresolved is the lack of a proper disease response monitoring index. Discrepancy between tumor-specific antigen (tumor marker) levels, imaging studies and efficacy has been shown in some trials (47). Because tumor markers and imaging studies are the two main indices to monitor the disease status, the discrepancy leads to difficulty in interpretation. For prostate cancer, an ideal cancer model for gene therapy because of the ease of local access for vector administration, any local stimulation (including massaging and injection itself) will increase the serum PSA level, for example. This makes interpretation of the tumor markers difficult in these patients.

Furthermore, for biological activity determination, patients are required to have multiple invasive tissue samplings (biopsies). This adds to the inconvenience and potential risk to patients. In addition, significant sampling bias may be reflected from the data

Fig. 1. Staged approach to cancer gene therapy trials.

Infra-venous • Solid tumor met.

Fig. 1. Staged approach to cancer gene therapy trials.

obtained from biopsies, especially when the tumors are treated with local injections. Thus, a monitoring system for biological activity and disease status that can follow patients with minimal risks, convenience and cost while obtaining high-quality, realtime data is badly needed.

In cases where tumor suppressor gene(s) are delivered to restore the normal function of those genes, it is also worth reconsidering the current concept of gene delivery in the context of transgene expression level. Although transgene expression can be documented by various methods, one must bear in mind that successful gene delivery (as tested by polymerase chain reaction [PCR] ) and/or transcription (mRNA expression) does not necessarily lead to functional protein expression. Hence, to monitor biological activities of this type of approach, more data must be obtained to realize the effect of gene restoration. In addition, the therapeutic level of transgene products that is high enough to induce significant antitumoral effect in patients must be determined.

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