Chemically modified non-antimicrobial tetracyclines are multifunctional drugs against advanced cancers

Abstract

Metastatic cancers account for more than 90% of cancer mortality. The metastasis of all cancers is critically mediated by enzymes that degrade extracellular matrix. Aggressive tumors are characterized by an imbalance between enzymes that degrade ECM and endogenous inhibitors of the enzymes. Matrix metalloproteinases (MMPs) make up the majority of ECM degrading enzymes implicated in cancer metastasis. The potent MMP inhibitory activities of tetracyclines, especially their chemically modified analogs, combined with their relatively well tolerated pharmacological profile, led several researchers to investigate their anticancer potential in a variety of cancers, including melanoma, lung, breast and prostate cancers. Chemically modified non-antibiotic tetracyclines (CMTs or COL) were tested using tumors of prostate, breast and melanomas. Some of these CMTs, notably, CMT-3 and CMT-308 significantly inhibited not only invasive potential and MMP activity, but also inhibited cell proliferation by inducing cell cycle arrest and apoptosis. CMT-3 and CMT-308 were significantly more potent than doxycycline or minocycline in inhibiting tumor cell-derived MMPs and inducing apoptosis in vitro and in vivo. CMT-3 (COL-3) showed potent inhibition of tumor growth in xenografts and in bone metastatic models of prostate cancer. Similar results were also reported in melanoma and breast cancer models. The mechanism by which CMTs kill tumor cells is via generation of hydroxyl free radicals ([OH](-)) which permeate and depolarize mitochondria, which in turn activates caspase mediated apoptosis. Analysis of tumor tissues from CMT-3 treated rats demonstrated reduction in angiogenesis and increase in apoptosis; both emerged as mechanisms of CMT action. These observations led to testing the efficacy of CMT-3 in human clinical trials against several types of cancer with significant outcomes, which are described in the next chapter of this issue.

Fig 1. CMT-3 Inhibition of experimental prostate cancer metastasizing to bone and lung in the Dunning MAT-LyLu model

The MAT Ly Lu tumor cells were injected i.v following vena cava clamping with a mini surgical bulldog clamp. As described in the text, animals were orally dosed with CMT-3 or vehicle -7 days (Group 1),+ 1 day (Group 2), or +3 days (Group 3) and vehicle-only (Group 4) [Panel B]. In an independent experiment, MAT LyLU cells transfected with EGFP were injected as shown in Fig. 1A and were detected as dispersed in femoral bones (Fig. 1C and D). Cells isolated from marrow plugs were cultured to confirm tumor cells (data not shown).

Fig. 2. A suggested model for CMT-3 induced tumor cell cytotoxicity

CMT-3 induced cytotoxicity is due to both cell cycle arrest at G1/S interphase and free-radical induced-mitochondrial permeability changes and caspase activation. Both, cytosolic and ER mediated caspase activation.