Missing Selectivity of Targeted 4β-Phorbol Prodrugs Expected to be Potential Chemotherapeutics

Abstract

Targeting cytotoxic 4β-phorbol esters toward cancer tissue was attempted by conjugating a 4β-pborbol derivative with substrates for the proteases prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA) expressed in cancer tissue. The hydrophilic peptide moiety was hypothesized to prevent penetration of the prodrugs into cells and prevent interaction with PKC. Cleavage of the peptide in cancer tumors was envisioned to release lipophilic cytotoxins, which subsequently penetrate into cancer cells. The 4β-phorbol esters were prepared from 4β-phorbol isolated from Croton tiglium seeds, while the peptides were prepared by solid-phase synthesis. Cellular assays revealed activation of PKC by the prodrugs and efficient killing of both peptidase positive as well as peptidase negative cells. Consequently no selectivity for enzyme expressing cells was found. KEYWORDS: 4β-Phorbol ester, protease-assisted targeting, targeted chemotherapy, prodrug, prostate-specific antigen. prostate-specific membrane antigen P rostate cancer (PCa) is a major cause of death by cancer in men in high-income countries. 1 In the initial stage, PCa mainly consists of cells that are androgen-dependent, and the growth can be retarded by hormone therapy. 2 Unfortunately, in later stages hormone refractory cells dominate (castrationresistant prostate cancer, CRPC). 2,3 At this stage the use of common chemotherapeutics is complicated by the slow proliferation of the cancer tissue, since chemotherapeutics like taxanes, doxorubicine, or vincristine target the proliferative stages of cancer. Thus, selectivity is obtained by the faster division rate for cancer cells. 3−6 Therefore, an urgent need for drugs against late-stage PCa exists.Preclinical evidence supports the idea that drugs targeting protein kinase C (PKC) may be useful in treatment of CRPC. 7 The PKC family comprises ten serine/threonine kinases, which can be divided into three groups: (i) conventional PKC (cPKCs: -α, -βI, -βII, and -γ), (ii) novel PKCs (nPKCs: -δ, -ε, -θ, and -η), and (iii) atypical PKCs (aPKCs: -ζ, -ι, and λ). Expression and function of different PKC isoforms are context-

Protein Kinase A–Mediated Effects of Protein Kinase C Partial Agonist 5-(Hydroxymethyl)Isophthalate 1a3 in Colorectal Cancer Cells

“…Cell viability was determined by the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) reduction assay, and cell membrane integrity using lactate dehydrogenase (LDH) release assay from cell culture media, as previously described (Tarvainen et al, 2020). In brief, the cells seeded onto 96-well plates (10 000 cells per well, in serum-supplemented media), were incubated with the compounds in normal cell culture medium for 24 h. The LDH assay was carried out with 50 µL samples of culture medium from each well.…”

The Role of Phorbol Diesters in Mediating Human Placental Aromatase Cytochrome P450 Activity

“…Cytotoxic 4-phorbol esters were used to target cancerous tissues. The findings demonstrated that the phorbol prodrug effectively eliminated peptidase-positive and -negative cells and activated PKC [41]. Tsai et al studied the activation of PKC by phorbol esters using platelet aggregation as a model.…”

Targeting Toxins toward Tumors

“…However, the results obtained in clinical phase II trial did not meet the expectations, and hence the drug was not marketed [ 58 ]. A major problem might be that the prodrug, despite its five negative charges on its peptide side chains, has been found to be able to penetrate cell membranes in benign cells, thereby causing unspecific toxicity [ 59 ].…”