This post is prompted by Ed Miller's:
"Ten days of the antibiotic clendamycin lowered my PSA for 3 months."
I remembered seeing some PCa-tetracycline studies. (No PCa hits for "Clindamycin".)
The matrix metalloproteinase [MMP] family comprise enzymes that break down extracellular matrix proteins. The matrix holds cells in place & MMPs are essential for metastasis. Key MMPs: MMP-2 (gelatinase A) & MMP-9.
The following studies are not complete, but probably sufficient to grasp the possibilities - particularly [2b].
 (1997 - Canada)
"Matrix metalloproteinases are implicated in various steps of development of metastasis, through their ability to degrade the extracellular matrix. Increased matrix metalloproteinase activity of tumor cells has been associated with a higher metastatic potential. Inhibitors of metalloproteinases have been shown to effectively reduce or prevent the formation of metastases. The family of tetracyclines is able to inhibit matrix metalloproteinase activity through chelation of the zinc ion at the active site of the enzyme."
"Using tumor cell lines relevant to bone metastases, i.e. PC-3, ..., we showed that tetracycline and derivatives of tetracycline, namely doxycycline and minocycline, also induced cytotoxicity. The effective concentrations are relatively high for plasma, but are clinically achievable in the bone, since tetracyclines are osteotropic."
[2a] (1998 - U.S.)
"The matrix metalloproteinases (MMPs) play a significant role in the growth, invasion and metastasis of many tumors, including those of the prostate. We previously demonstrated that doxycycline, a synthetic tetracycline, inhibits MMPs and cell proliferation and induces apoptosis in several cancer cell lines. We also demonstrated that in an in vivo model of metastatic breast cancer in athymic mice doxycycline inhibits tumor size and regrowth after resection. In the present study, gelatinolytic activity in the human prostate cancer cell line, LNCaP, was suppressed and significant inhibition of cell growth occurred after exposure to 5 or 10 microg/ml of doxycycline, while cell growth was normal in untreated cells."
[2b] (2015 - U.K. / U.S.)
"Antibiotics that target mitochondria effectively eradicate cancer stem cells, across multiple tumor types: Treating cancer like an infectious disease"
"... doxycycline is relatively attractive as a new anti-cancer agent, as it has a long half-life systemically and has been used successfully for the long-term treatment of patients with urinary tract infections (UTI), prostatitis or acne, for extended periods of time, of up to 4-to-6 months or more (200 mg per day). Doxycycline also encourages the growth of normal stem cells, has anti-inflammatory properties, and even increases lifespan, in certain experimental contexts [12-14]. Thus, the toxic side effects of anti-cancer therapy would be minimized.
"Doxycycline has also been used in human tumor xenografts and other animal models to significantly reduce tumor burden and even metastatic cancer cell growth [15-20]. For example, in pancreatic tumor xenografts (with PANC-1 cells), doxycycline treatment reduced tumor growth by ~80% . In a xenograft model of breast cancer bone metastasis (with MDA-MB-231 cells), doxycycline treatment reduced bone and bone-associated soft-tissue tumor mass by >60% and ~80%, respectively . However, its anti-cancer activity was attributed to the inhibition of matrix-metalloproteinases (MMPs), rather than the targeting of mitochondrial biogenesis, and doxycycline has not been previously implicated in the selective eradication of cancer stem cells [15-20].
"Our results are consistent with the previous finding that metformin, a widely used anti-diabetic drug, which functions as a mitochondrial inhibitor, can also be used to selectively target CSCs [21, 22]. Metformin functionally inhibits OXPHOS by targeting complex I of the electron transport chain and can even induce lactic acidosis, as a lethal side effect [21, 22]. As a result, the use of antibiotics, such as doxycycline, may provide a safer and far more effective alternative to anti-cancer therapy with metformin."
[3a] (2014 - Germany)
"Minocycline was identified to potently inhibit cell growth, at concentrations within the range of tissue levels readily reached under standard therapeutic conditions."
"Our findings add to the growing body of evidence for the many pleiotropic actions of minocycline. In view of the striking effects of minocycline on cell growth in PCA cell lines in vitro and its relatively safe side effect profile, the use of minocycline for targeting PCA should be timely clinically evaluated."
 CMT-3. (Non-antimicrobial Tetracycline analog.)
[4a] (2001 - U.S.)
"Tetracyclines (TCs) and their non-antimicrobial analogs (CMTs) have therapeutic potential to inhibit tissue destructive disease processes, such as cancer invasion and metastasis, by inhibiting certain matrix metalloproteinases. Enhanced matrix metalloproteinase-2 (MMP-2; gelatinase A) activity has been correlated to cancer invasiveness, and membrane type MMP (MT1-MMP) expressed by tumor cells is involved in localizing and activating pro-MMP-2, a pathway believed to mediate cancer induced tissue breakdown."
"CMT-3 ... has been shown to experimentally suppress prostate cancer, colon adenocarcinoma and melanoma invasiveness in cell culture and to inhibit tumor growth and metastasis in vivo and was used in the current in vitro study."
"CMT-3 at final concentrations of 5--20microM inhibited MT1-MMP gelatinolytic and caseinolytic activity, blocked MT1-MMP activation of pro-MMP-2, and decreased invasiveness"
"The inhibition of MT1-MMP by CMT-3 may partially explain the inhibition of cancer cell -mediated tissue breakdown and invasiveness by this non-antimicrobial tetracycline analog."
[4b] (2011 - U.S.)
"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."
"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 ..."