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The chemical derivatization of biologically active microbial metabolites continues to be a promising approach to the identification of new drugs. We recently synthesized the novel antiproliferative compound SDZ 281-977, 5-[2-(2,5-dimethoxy-phenyl)ethyl]-2-hydroxy-benzoic acid methylester, a derivative of the EGF receptor tyrosine kinase inhibitor lavendustin A. Here we report on our studies of the anticancer efficacy and the mode of action of SDZ 281-977. The growth of both the human pancreatic tumor cells MIA PaCa-2 and the human vulvar carcinoma cells A431 was inhibited in the low micromolar range. Tumors from these cells were induced in nude mice and were shown to respond to orally or intravenously administered SDZ 281-977. In contrast, no antitumor effect was detected in rats bearing dimethylbenzanthracene-induced mammary tumors. Studies in mice indicated that SDZ 281-977 was neither immunosuppressive nor hematosuppressive at doses effectively inhibiting tumor growth. Surprisingly, the mode of action of SDZ 281-977 apparently does not involve inhibition of EGF receptor tryosine kinase, because, in contrast to lavendustin A, SDZ 281-977 failed to inhibit this enzyme in a cell-free assay. The mechanism of the antiproliferative effect can be explained on a cellular level by the ability of the compound to arrest cells in mitosis. SDZ 281-977 is thus the first example of an antimitotic agent derived from the potent tyrosine kinase inhibitor lavendustin A. The therapeutic potential of SDZ 281-977 is enhanced by the fact that it is not subject to multidrug resistance, because tumor cells expressing the multidrug resistance phenotype were as sensitive to SDZ 281-977 as their nonresistant counterparts. In conclusion, SDZ 281-977 represents a novel lavendustin A derivative with potent antiproliferative properties in vitro and in vivo that may be explained on the basis of its antimitotic effects. SDZ 281-977 may be a candidate drug for the treatment of selected cancers, including those expressing the multidrug resistance phenotype.

The somatostatin analogue octreotide (SMS 201-995) exerts potent anti-proliferative effects in a number of experimental cancer models. Here we report on the inhibitory effect of octreotide in combination with the chemotherapeutic agents mitomycin C, doxorubicin, 5-fluorouracil, or taxol on the growth of AR42J pancreatic cancer cells in vitro. The dose-dependent anti-proliferative effects of mitomycin C, doxorubicin and taxol were synergistically enhanced by octreotide. Combinations of octreotide and 5-fluorouracil resulted either in additive or, at high concentrations of the chemotherapeutic agent, in synergistic interactions. Combined treatment with doxorubicin and octreotide was also studied for time dependency and potential efficacy in tumour-bearing animals. Pretreatment (24 h) with doxorubicin resulted in clear synergy. However, pretreatment with octreotide 24 h prior to addition of doxorubicin resulted only in an additive interaction. It was shown in AR42J-tumour-bearing nude mice that the combination of doxorubicin and octreotide was well tolerated. Tumour growth was inhibited to 9% of controls, compared with 44% in the doxorubicin alone arm (day 14 of treatment). Our in vitro and in vivo interaction studies suggest that octreotide potentiates the effect of various chemotherapeutic agents in a synergistic or additive manner.

The active partial structure of the potent tyrosine kinase inhibitor lavendustin A was derivatized in the search for novel agents against cellular proliferation. The antiproliferative potential of the new derivatives was determined using the human keratinocyte cell line HaCaT as the primary test system. Whereas the lavendustin A partial structure is ineffective in inhibiting cell proliferation, esterification of its carboxylic acid function leads to measurable antiproliferative activity. Additional O-methylation of the 2,5-dihydroxyphenyl moiety yields activity in the micromolar range. Further substantial increases in activity are achieved by replacing the nitrogen with oxygen and carbon within the 2,5-dimethoxyphenyl series (but not within the 2,5-dihydroxyphenyl analogs) leading to 5-[2-(2,5-dimethoxyphenyl) ethyl]-2-hydroxybenzoic acid methyl ester (13) as the most potent analog identified to date. These increases in antiproliferative activity are paralleled, however, by the disappearance of activity against the epidermal growth factor receptor-associated tyrosine kinase, suggesting another mechanism of action.

Somatostatin (SRIF) is a cyclic tetradecapeptide hormone initially isolated from ovine hypothalami. It inhibits endocrine and exocrine secretion, as well as tumor cell growth, by binding to specific cell surface receptors. Its potent inhibitory activity, however, is limited by its rapid enzymatic degradation and the consequent short plasma half-life. Octreotide is a short SRIF analog with increased duration of action compared to SRIF. Octreotide is approved for the treatment of acromegaly, amine precursor uptake and decarboxylation-omas, complications of pancreatic surgery and severe forms of diarrhea. Preclinical studies have focussed on the anticancer effects of octreotide and the related SRIF analogs BIM 23014 and RC-160. In vitro at nanomolar concentrations, these analogs inhibit the growth of tumor cells that express high affinity SRIF receptors. Accordingly, SRIF analogs, such as octreotide, potently inhibit the growth of SRIF receptor-positive tumors in various rodent models, and, in particular, xenotransplanted human tumors in nude mice. The range of cancers susceptible to octreotide and related SRIF analogs includes mammary, pancreatic, colorectal and lung malignancies. Moreover, an indirect antiproliferative effect of SRIF analogs is achievable in SRIF receptor-negative tumors, whose growth is driven by factors (gastrin, insulin-like growth factor-1, etc.) that are downregulated by SRIF. The use of radiolabeled somatostatin analogs represents a new diagnostic approach. [111In-DTPA]octreotide was developed for gamma camera imaging of SRIF receptor-positive malignancies, such as gasteroenteropancreatic tumors. Visualization of SRIF receptor-positive tumors in humans is emerging as an important methodology, both in tumor staging and predicting therapeutic response to octreotide. Recently, five SRIF receptor subtypes (SSTR1-5) have been cloned, all of which bind SRIF with high affinity. In contrast, SRIF receptor subtypes 1-5 have different binding profiles for short SRIF analogs. Octreotide, SSTR5, show moderate affinity for SSTR3 and fail to bind with high affinity to the other subtypes (SSTR1 and 4). Accordingly, the oncological profile of these three analogs is apparently similar. In conclusion, somatostatin analogs are a promising class of compounds for diagnosis and treatment of cancer. Current work is focussed on the identification of further SRIF receptor subtype-selective analogs with potential in oncology.

Administration of 2,3-dihydro-1H-pyrazole[2,3a]imidazole (IMPY, 150 mg/kg) followed 8 hr later by injection of deoxyadenosine/erythro-9-(2-hydroxyl-3-nonyl)adenine (dAdo/EHNA, 175 mg/17.5 mg/kg) on days 2, 3, 6, and 7 increased the mean survival time of L1210 tumor bearing mice (210%). The sequential treatment was more efficacious than the simultaneous administration of these drugs. Administration of IMPY or dAdo/EHNA, alone, at the same doses as in the combination, did not prolong the life-span of tumor bearing mice. To determine the basis for the increased survival due to the sequential treatment with IMPY and dAdo/EHNA, cell cycle analysis and deoxyribonucleoside triphosphate concentrations were measured. Cytotoxicity of IMPY and dAdo/EHNA is known to be achieved through the inhibition of ribonucleotide reductase. IMPY is a specific inhibitor of the nonheme-iron subunit of ribonucleotide reductase, whereas deoxyadenosine in the presence of the adenosine deaminase inhibition, EHNA, is converted to deoxyadenosine 5'-triphosphate (dATP), which is a specific inhibitor of the effector-binding-subunit of ribonucleotide reductase. Our studies showed that L1210 cells accumulated in early S-phase, whereas intracellular dATP and deoxyguanosine triphosphate (dGTP) pools were depleted 8 hr after IMPY administration. dAdo/EHNA administration 8 hr after IMPY injection caused an increase in the intracellular concentration of dATP while maintaining the depletion of the dGTP pool and prolonged the S-phase as compared to the administration of IMPY alone.

2,6-Diaminopurine (DAP) and 2,6-diaminopurine 2'-deoxyriboside (DAPdR) are analogs of adenine and deoxyadenosine, respectively. It was the purpose of this study to compare these analogs under identical conditions in order to define their inhibitory properties and the underlying mechanism in L1210 mouse leukemia cells. In a 5-day cell growth experiment, DAP exerted a significantly stronger antiproliferative effect than DAPdR. Correspondingly, colony formation of L1210 cells in soft agarose was inhibited by DAP to a greater extent than by DAPdR. A differential distribution of L1210 cells in the cell cycle resulted from an exposure to DAP and DAPdR. While DAPdR arrested cells in the G1/G0 phase of the cell cycle, DAP appeared to lead to an accumulation of G2/M cells. The diaminopurines were combined with modulatory agents to test the antiproliferative action of the combinations. Deoxycytidine partially rescued the cells from the growth inhibitory action of DAPdR without affecting the growth of DAP-treated cells. When adenine was used, the antiproliferative effect of DAPdR was slightly enhanced while the effect of DAP was completely abolished. 8-Aminoguanosine, a specific inhibitor of purine nucleoside phosphorylase, synergistically potentiated the cytostatic effect of DAPdR. However, this inhibitor did not alter DAP effects. At the biochemical level, the target of DAPdR was ribonucleotide reductase which was in line with a drastic expansion of the dGTP pool in DAPdR-treated cells. In cells exposed to DAP, high levels of DAP riboside triphosphate were measured; concomitantly, the ATP level dropped markedly. Enzymological studies revealed that DAPdR is an excellent substrate of adenosine deaminase giving rise to the formation of deoxyguanosine. DAP was found to be activated in the purine nucleoside phosphorylase reaction and in a phosphoribosyl-pyrophosphate-dependent reaction. The data from this comparative study suggest that DAPdR and DAP possess different toxicity mechanisms. DAPdR and DAP possess different toxicity mechanisms. DAPdR acts as a precursor of deoxyguanosine, and DAP is metabolically activated to DAP-containing ribonucleotide analogs. These different metabolic routes seem to account for the different effects of DAP and DAPdR at the cellular level.