New apratoxins of marine cyanobacterial origin from guam and palau
Analogues of the potent cytotoxin apratoxin A are described.
Introduction
Marine cyanobacteria are known for their great biosynthetic potential and ability to produce a wide range of cytotoxic compounds, some with possible therapeutic applications such as curacins1 and dolastatins.2 As a result of our ongoing search for new antitumor compounds from marine cyanobacteria, we recently described apratoxin A (1) as the most potent cytotoxin produced by a variety of the cyanobacterium Lyngbya sp. from Finger's Reef, Apra Harbor, GU, USA.3 Its mode of action is still unknown at this time. To initiate SAR studies we searched for natural analogues of 1 and discovered apratoxin B (2) in minor amounts in the largest recollection of this organism. Populations of the same cyanobacterium, recognized by their dark reddish color, strong attachment to the reef substrate, and predictable coexistence with the red alpheid shrimp Alpheus frontalis, were also chemically investigated at other sites around the island of Guam and in Palauan waters. These organisms displayed similar natural products chemistry, including the production of apratoxin A (1). A collection from Short Dropoff, Palau, yielded the analogue apratoxin C (3) in addition to 1. A semisynthetic compound, E-dehydroapratoxin A (4), and the new natural apratoxins 2 and 3 were evaluated for their cytotoxicity compared to apratoxin A (1). Comparison of the different apratoxin producers from Guam and Palau on the genetic level has been initiated.
Section snippets
Structure determination
The structures of compounds 2–4 were elucidated by interpretation of spectroscopic data and analysis of degradation products. HRFABMS analysis and NMR spectra indicated a molecular formula of C44H67N5O8S for compound 2, one methylene unit less than for 1. The 1H NMR spectrum of 2 displayed only one N-methyl singlet (δ 2.64 in CDCl3) instead of two as in the corresponding spectrum of 1, accounting for the mass difference. Further 1D NMR (Table 1) and routine 2D NMR analysis in CDCl3 revealed an
Conclusion
In summary, we have described two new natural analogues, 2 and 3, of the potent cytotoxin apratoxin A (1) from Micronesian cyanobacterial collections. SAR studies have been initiated by comparing their in vitro cytotoxicity with the ones of 1 and its dehydration product 4. This information could be helpful for synthetic and medicinal chemists targeting this compound group. The biosynthesis of the apratoxins is under investigation.
General
All NMR experiments were run on a Varian Unity Inova 500 spectrometer. 1H and 13C NMR spectra were recorded in CDCl3 at 500 and 125 MHz, respectively, using residual solvent signals as internal references. The HSQC experiments were optimized for 1JCH=140 Hz, and the HMBC experiments for nJCH=7 Hz. HRMS were obtained by FAB in the positive mode using a VG-ZAB spectrometer. IR spectra were recorded on a Perkin-Elmer 1600 Series FT-IR. UV spectra were obtained on a Hewlett-Packard 8453 spectrometer.
Acknowledgments
Funding was provided by NCNPDDG grant CA53001 from the National Cancer Institute. The upgrade of the 500 MHz NMR spectrometer used in this research was funded by grants from the CRIF Program of the National Science Foundation (CHE9974921) and the Elsa U. Pardee Foundation. We thank the Division of Marine Resources of the Republic of Palau and the Koror State Government for marine research permits. Jason Biggs and Dr. Edwin Cruz-Rivera helped collecting the cyanobacterium in Palau. Cytotoxicity
References (8)
- et al.
Chem. Biol.
(2000) - et al.
J. Org. Chem.
(1994) - et al.
J. Nat. Prod.
(2001) - et al.
J. Am. Chem. Soc.
(2001)