Prostatic cancer with bone metastases: serum alkaline phosphatase (SAP) as a predictor of response and the significance of the SAP "flare".
The predictive value of serum alkaline phosphatase (SAP) and of prostatic acid phosphatase (PAP) for response to treatment (NPCP criteria) was retrospectively assessed in patients with bone metastases from prostate cancer. Fifty-one patients had SAP measured at the start of treatment and at 1 and 2 months. In 31 of these, corresponding PAP levels were also available at each time point. SAP/PAP profiles at 2 months were classified as "increased" (increment 15% or greater), "decreased" (reduction greater than 15%) or "stable", compared with baseline levels. An additional category, SAP "flare", was also identified (SAP increment greater than 15% at 1 month, with subsequent fall at 2 months). There was a strong association between the SAP profile at 2 months and the response category, whereas the PAP profile at 2 months was more weakly associated. Using results from the 31 patients with both SAP and PAP profiles, the level of SAP was significantly better in predicting the category of response (SAP: sensitivity 94%, specificity 79%; PAP: sensitivity 53%, specificity 57%). An SAP "flare" was associated with response in 8 of 12 patients. An increase in SAP at 1 month is therefore a poor guide to progressive disease and should not be used in isolation to discontinue treatment early. The SAP profile is of value as an earlier predictor of response than X-rays or bone scans and is more reliable than the PAP profile in monitoring patients with prostate cancer and bone metastases.