EVALUATION OF CREATINE ANALOGS AS A NEW CLASS OF ANTICANCER AGENTS USING FRESHLY EXPLANTED HUMAN TUMOR-CELLS

Background: The creatine kinase (CK) isozymes and their substrates, cr eatine and creatine phosphate, are believed to play a pivotal role in energy transduction in tissues with large, fluctuating energy demands, such as skeletal muscle, heart, and brain. This enzyme system may als o be involved in the process of cellular transformation. Inhibition of tumor cell growth by creatine analogues has been observed and may be due to the ability of these analogues to impair cellular energy genera tion and utilization. Purpose: An in vitro human tumor colony-forming assay was used to predict the clinical usefulness of creatine analogue s as anticancer agents. Methods: The ability of cyclocreatine (1-carbo xymethyl-2-iminoimidazolidine) and homocyclocreatine (1-carboxyethyl-2 -iminoimidazolidine) to inhibit the growth of cells prepared from tumo r samples taken directly from patients was evaluated by quantitative m easurement of colony formation in a soft-agar cell culture assay syste m. Cyclocreatine was tested in this human tumor colony-forming assay a t concentrations ranging from 0.067 to 20 mM against 128 tumor samples , 51 of which formed colonies in the assay and were considered evaluab le. Homocyclocreatine was similarly tested at concentrations from 0.2 to 20 mM against 139 tumor samples; 54 were considered evaluable. The colony-forming assay was also used to compare the efficacy of the crea tine analogues to representatives from the six major classes of standa rd chemotherapeutics (alkylating agents, antimetabolites, DNA intercal ators, platinum compounds, topoisomerase inhibitors, and tubulin-inter acting agents). In addition, CK levels were measured in 192 tumor samp les that were taken from 166 patients. Results: Cyclocreatine and homo cyclocreatine, at concentrations previously achieved in animal tissues (7-20 mM), had antitumor activity against 19% and 50%, respectively, of tumor samples that formed colonies in the assay. Cyclocreatine was effective against a subset of tumors sensitive to homocyclocreatine (P = .023; Fisher's exact test), which was the more potent creatine anal ogue in this assay (P<.001; McNemar's test). No relationships were see n between tumor samples sensitive to the creatine analogues and those sensitive to standard chemotherapeutics. Pairwise Wilcoxon rank sum te sts indicated that CK activity was significantly higher in tumors with any growth in the colony assay compared with tumors that did not grow (P<.025). Conclusions: The creatine analogues, cyclocreatine and homo cyclocreatine, effectively reduced colony formation of freshly explant ed human tumor cells. The mechanism of action or resistance to these c ompounds seems to differ from those of standard chemotherapeutics. Imp lications: Creatine analogues that may alter the energy status of the tumor cell potentially represent promising new anticancer agents that function through a unique mechanism.