John R. MacDonald

Effects on DNA integrity and apoptosis induction by a novel antitumor sesquiterpene drug, 6-hydroxymethylacylfulvene (HMAF, MGI 114)

6-Hydroxymethylacylfulvene (HMAF, MGI 114) is a new alkylating antitumor sesquiterpenoid with promising and often curative antitumor activity in vivo. This study examined the ability of the drug to damage cellular DNA, induce apoptosis, and affect the cell cycle of CEM human leukemia cells. No bifunctional lesions, interstrand DNA cross-links or DNA-protein cross-links were seen (by alkaline sedimentation and K+/SDS precipitation, respectively) when using up to 50 microM HMAF. The drug possibly formed some monoadducts, as DNA from drug-treated cells impeded primer extension by Taq polymerase, although only partial inhibition was seen even at 200 microM HMAF. HMAF also induced secondary lesions in cellular DNA, single-strand breaks that were detectable (by nucleoid sedimentation and alkaline sucrose gradient analysis) after a 4-hr treatment at HMAF levels as low as 2 microM, comparable to the growth inhibition IC50 value (1.7 microM). A post-treatment incubation of cells in drug-free medium generated substantial amounts of DNA double-stranded fragments of several kbp, suggesting apoptotic fragmentation (>30% of total DNA following treatment with 20 microM HMAF and a 17-hr post-treatment incubation). Chromatin condensation (by ultrastructural analysis) and induction of sub-G1 particles and apoptotic strand breakage (by multiparametric flow cytometry) confirmed induction of apoptosis by HMAF. HMAF preferentially inhibited DNA synthesis (IC50 approximately 2 microM), which is consistent with an S phase block, observed by cell cycle analysis. The pattern of apoptotic DNA fragmentation, inhibition of DNA synthesis, and blockage in the S phase suggests that these events play a role in the antiproliferative activity of HMAF.

Differential cytotoxicity and induction of apoptosis in tumor and normal cells by hydroxymethylacylfulvene (HMAF)

This investigation compared the effects of hydroxymethylacylfulvene (HMAF), a novel antitumor drug with alkylating properties, in eight human tumor (prostate, colon, and leukemia) cell lines, and five human normal (prostate and renal proximal tubule epithelial, colon mucosa, fibroblasts, and endothelial) cell lines. Drug-induced growth inhibition paralleled the uptake of HMAF into both tumor and normal cells, although normal cells were 3- to 4-fold more tolerant to the accumulated drug. In both tumor and normal cells, approximately two-thirds of internalized [(14)C]HMAF-derived radioactivity was bound covalently to macromolecules. Trypan blue exclusion and cell counts indicated that HMAF was cytotoxic in tumor but cytostatic in normal cells. Correspondingly, profound apoptosis was detected in all tumor cell lines examined. A 4-hr treatment with HMAF followed by 20-hr post-incubation induced a potent DNA fragmentation in nearly all tumor lines. Apoptosis-resistant PC-3 and HT-29 cells underwent significant DNA fragmentation after 24 hr of continuous treatment with HMAF. In contrast to tumor cell lines, marginal or very low levels of apoptosis were detected in the normal cells even after prolonged treatments with HMAF at concentrations that exceeded 15- to 800-fold the GI(50) values in tumor cells. This resistance of normal cells to apoptosis could not be accounted for by differences in drug accumulation or drug covalent binding to macromolecules. The qualitatively different responses of the tumor and normal cells studied suggest a greater tolerance of normal cells to HMAF-macromolecular adducts. The demonstrated differential cytotoxic/cytostatic and apoptotic effects of HMAF can be of significance for the clinical use of this promising new agent.

Drug uptake and cellular targets of hydroxymethylacylfulvene (HMAF).

Hydroxymethylacylfulvene (HMAF, MGI 114) is a novel antitumor drug and a potent pro-apoptotic agent that has the potential to alkylate cellular nucleophiles. The objective of these studies was to characterize drug uptake and cellular targets for drug binding in human leukemia CEM cells. The uptake of [14C]HMAF had two components: a rapid phase (0-10 min) and a slow phase. At 10 microM drug (37 degrees), the rapid and slower phase amounted to 0.86 and 0.13 pmol/min/10(6)cells, respectively. HMAF uptake was inhibited 82% by low temperature (4 degrees) at 4 hr. Cell-associated HMAF localized to nuclear (50%), cytoplasmic (37%), and membrane fractions (10%). Continued drug uptake appeared to be driven by covalent binding to cellular macromolecules. Approximately 1/4 and 2/3 of cell-associated HMAF formed covalent adducts after 10 min and 4 hr, respectively, as found by perchloric acid precipitation. Drug adducts were not readily reversible; 77% of the covalently bound radiolabel was retained by the cells 20 hr after drug treatment. Combinations of DNase, RNase, and proteinase K with perchloric acid precipitation showed that approximately 60, 30, and 10% of the covalently bound drug was associated with the protein, DNA, and RNA fractions, respectively. Incubation of 100 microM [14C]HMAF (24 hr) with purified DNA, serum albumin, thioredoxin, and thioredoxin reductase resulted in 6, 22, 14, and 11 pmol [14C]HMAF/microg DNA or protein, respectively. Results indicate that multiple targets for HMAF binding may contribute to the pro-apoptotic and antiproliferative action of the drug.

Inhibiting tumor growth by targeting tumor vasculature with galectin-1 antagonist anginex conjugated to the cytotoxic acylfulvene, 6-hydroxylpropylacylfulvene.

Targeted delivery of therapeutic drugs promises to become the norm to treat cancer. Here, we conjugated the cytotoxic agent 6-hydroxypropylacylfulvene (HPAF) to anginex, a peptide that targets galectin-1, which is highly expressed in endothelial cells of tumor vessels. In a human ovarian cancer model in mice, the conjugate inhibited tumor growth better than equivalent doses of either compound alone. Immunofluorescence on tumor tissue demonstrated that the conjugate, like parent anginex, selectively targeted tumor vasculature and inhibited tumor angiogenesis. Increased activity from the conjugate further suggests that HPAF retains at least some of its normal cytotoxic activity when linked to anginex. More importantly perhaps is the observation that the conjugate abrogates apparent systemic toxicity from treatment with HPAF. This work contributes to the development of tumor vascular targeting agents against cancer in the clinic.

Ovarian tumor growth regression using a combination of vascular targeting agents anginex or topomimetic 0118 and the chemotherapeutic irofulven

Combination of chemotherapeutic agents and angiogenesis inhibitors is now commonly employed in the clinic to treat cancer. Here, we used angiostatic agents anginex and 0118, in combination with the chemotherapeutic irofulven, to treat human ovarian tumor xenografts in mice. General linear mixed models were used to statistically analyze tumor growth curves. Overall, combination of a low, non-toxic dose of irofulven with either angiogenesis inhibitor was more effective at inhibiting tumor growth than any of the single agent therapies. For example, the anginex/irofulven and 0118/irofulven combinations inhibited tumor growth relative to controls by 92% (p<0.0001) and 96% (p<0.0001), respectively, with the 0118/irofulven combinations yielding 100% complete responses. This study suggests that combination therapy of 0118 or anginex and irofulven may be highly effective in the clinical setting.

Targeting Apoptosis by Hydroxymethylacylfulvene in Combination with Gamma Radiation in Prostate Tumor Cells

Abstract Woynarowska, B. A., Roberts, K., Woynarowski, J. M., MacDonald, J. R. and Herman, T. S. Targeting Apoptosis by Hydroxymethylacylfulvene in Combination with Gamma Radiation in Prostate Tumor Cells. Hydroxymethylacylfulvene (HMAF) is a novel agent with alkylating activity and is a potent inducer of apoptosis that is currently undergoing Phase II clinical trials for prostate cancer. This study explored the pro-apoptosis and anti-proliferative potential of HMAF in combination with γ radiation in human prostate tumor cell lines. Apoptosis was assessed based on the generation of fragmented DNA, a terminal transferase flow cytometry assay, and cell morphology. In each of the tumor cell lines examined, radiation alone induced a marginal level of apoptosis, even after a prolonged 48-h incubation after exposure. In contrast, HMAF alone was a potent inducer of apoptosis in prostate tumor cells but not in normal cells. Marked levels of apoptosis in tumor cells were also observed for the combination of HMAF with γ radiation. When drug treatment preceded irradiation, at least additive levels of apoptosis were observed in both androgen-responsive and androgen-independent cells. The combined treatment with ionizing radiation and HMAF reduced the radiation dose needed for the same level of clonogenic survival up to 2.5-fold. The potentiation of apoptosis and reduction in the clonogenic survival of tumor cells occurred at HMAF concentrations lower than that which reduced survival to 10% and at doses up to 6 Gy. No potentiation of apoptosis or clonogenic inhibition was noted in normal cells. These results suggest that the combination of HMAF with γ radiation may have clinical utility for treatments of prostate cancer.

Apoptosis induction by the dual-action DNA- and protein-reactive antitumor drug irofulven is largely Bcl-2-independent

The overexpression of Bcl-2 is implicated in the resistance of cancer cells to apoptosis. This study explored the potential of irofulven (hydroxymethylacylfulvene, HMAF, MGI 114, NSC 683863), a novel DNA- and protein-reactive anticancer drug, to overcome the anti-apoptotic properties of Bcl-2 in HeLa cells with controlled Bcl-2 overexpression. Irofulven treatment resulted in rapid (12hr) dissipation of the mitochondrial membrane potential, phosphatidylserine externalization, and apoptotic DNA fragmentation, with progressive changes after 24hr. Bcl-2 overexpression caused marginal or partial inhibition of these effects after treatment times ranging from 12 to 48hr. Both Bcl-2-dependent and -independent responses to irofulven were abrogated by a broad-spectrum caspase inhibitor. Despite the somewhat decreased apoptotic indices, cell growth inhibition by irofulven was unaffected by Bcl-2 status. In comparison, Bcl-2 overexpression drastically reduced apoptotic DNA fragmentation by etoposide, acting via topoisomerase II-mediated DNA damage, but had no effect on apoptotic DNA fragmentation by helenalin A, which reacts with proteins but not DNA. Irofulven retains its pro-apoptotic and growth inhibitory potential in cell lines that have naturally high Bcl-2 expression. Collectively, the results implicate multiple mechanisms of apoptosis induction by irofulven, which may differ in time course and Bcl-2 dependence. It is possible that the sustained ability of irofulven to induce profound apoptosis and to block cell growth despite Bcl-2 overexpression may be related to its dual reactivity with both DNA and proteins.

Antitumor activity of MGI 114 (6-hydroxymethylacylfulvene, HMAF), a semisynthetic derivative of illudin S, against adult and pediatric human tumor colony-forming units.

MGI 114 (6-hydroxymethylacylfulvene, HMAF) is a novel semisynthetic antitumor compound derived from the sesquiterpene mushroom toxin illudin S. Although illudins did not demonstrate significant activity as antiproliferative agents in tumor-bearing animals, several properties including its potent inhibition of DNA synthesis and a unique interaction with DNA led to a structure-activity-based synthetic effort to obtain analogs with improved therapeutic potential. MGI 114 was selected for further development based on its antitumor activity in numerous preclinical tests. MGI 114 was evaluated against adult and pediatric human tumors taken directly from cancer patients and cultured in a human tumor colony-forming assay (HTCFA) to assess the antitumor spectra, concentration-response relationship, schedule dependence and activity of this agent against tumors considered resistant to conventional anticancer drugs. Human tumor colony-forming units were treated with HMAF at concentrations of 0.001, 0.01, 0.1 and 1 microg/ml, both as a 1 h exposure and as a continuous 14 day exposure. A response was scored if there was 50% or less colony survival. In vitro response rates in the range of 50-80% were observed against tumor colony-forming units originating from carcinomas of the colon, kidney, breast, lung cancer, ovary and melanoma. MGI 114 also demonstrated antitumor activity against neuroblastoma colony-forming units. Antitumor activity was not influenced by exposure time as demonstrated by the similar responses rates obtained with the 1 h and continuous exposure at all concentrations tested. However, there was a significant positive concentration-response relationship to both exposure duration with responses increasing from below 10% at the lowest concentration to over 70% at the highest concentration, except for the pediatric tumors on the 1 h exposure for which this relationship was less apparent. At the higher concentration tested, MGI 114 displayed substantial antiproliferative effects in the range of 70% against tumor specimens resistant to classic cytotoxic agents including irinotecan, paclitaxel, 5-fluorouracil, cisplatin, doxorubicin and cyclophosphamide. These results demonstrate that MGI 114 exhibits a broad spectrum of antitumor activity against both adult and pediatric primary tumor colony-forming units in a concentration-dependent manner both at short and prolonged exposure duration. The substantial in vitro activity of MGI 114 at concentrations achievable in clinical trials, together with its activity against tumors resistant to classic standard cytotoxic drugs, justifies the further clinical evaluation of this unique agent.

MGI 114 : Augmentation of antitumor activity when combined with topotecan

PURPOSE 6-Hydroxymethylacylfulvene (HMAF; MGI 114; Irofulven) is a semisynthetic analogue of the mushroom toxin illudin S that has been shown to be a potent cytotoxic agent with an improved therapeutic index compared with its parent compound. The studies were conducted to evaluate the antitumor activity of MGI 114 as a single agent and in combination with topotecan against pediatric solid tumor cell lines and xenograft models. MATERIALS AND METHODS In vitro studies were designed to determine the cytotoxic potential of MGI 114 using the MTT assay and 13 pediatric tumor cell lines. In addition, combination in vitro studies were performed with MGI 114 and topotecan to generate isoeffect plots. Single agent and combination in vivo studies were also performed using MGI 114 against rhabdomyosarcoma and neuroblastoma xenograft models. RESULTS After a 1-hour exposure to MGI 114, the mean IC50 (+/-standard error of mean) for medulloblastoma, neuroblastoma, Ewing sarcoma/primitive neuroectodermal tumor, and rhabdomyosarcoma cell lines were 1.58+/-0.51, 1.60+/-0.82, 1.18+/-0.08, and 3.99+/-1.69 microg/mL, respectively. When tumor cells were exposed concurrently to MGI 114 and topotecan, evidence of synergy was observed in 10 of 12 (83%) cell lines. Single agent and combination in vivo studies with MGI 114 showed that this agent had substantial, and at times curative, antitumor activity against rhabdomyosarcoma and neuroblastoma xenograft tumors. CONCLUSIONS These data suggest that MGI 114 has significant efficacy as a single agent in preclinical studies against pediatric tumors. In addition, based on previous reports and the results presented here, combining MGI 114 with topotecan appears to be an attractive approach to the treatment of pediatric malignancies. After completion of the pediatric phase I studies of MGI 114, consideration should be given to phase II single agent and phase I combination studies with a topoisomerase I inhibitor such as topotecan or irinotecan.

Pilocarpine, a salivary gland radioprotectant, does not inhibit cytotoxic effect of γ-radiation on squamous cell carcinoma in vitro

PURPOSE Pilocarpine, a salivary stimulant, has been shown to protect salivary glands from gamma-radiation-induced damage during the radiotherapy of head and neck tumors. This study was performed to determine whether pilocarpine affects the survival of squamous carcinoma cells, line SCC-25, following gamma-radiation treatment. METHODS AND MATERIALS The survival of squamous carcinoma tumor cells, line SCC-25, following the exposure of cells to pilocarpine at concentration of 0-100 ng/ml given for 0-1 h prior to radiation at dose of 0-20 Gy was determined by an in vitro colony-formation assay. RESULTS The survival fractions of SCC-25 cells were identical for the control and pilocarpine-treated samples at all tested conditions. Calculated Do and Dq values did not depend on the presence of pilocarpine and were not affected by the time of incubation prior to irradiation. CONCLUSION Pilocarpine, at clinically relevant concentrations, given to the SCC-25 cells 1 h prior to or at the time of irradiation did not affect survival of SCC-25 cells in vitro. Pilocarpine does not sensitize or protect these tumor cells from the effects of y-radiation, suggesting that this agent should not compromise the tumoricidal effects of radiotherapy.