Despite well over half a century of research aimed at identifying new analgesics, analgesics derived from morphine continue to serve as the front-line treatment for severe pain. While the clinical use of opiates is widespread, they suffer from a number of serious drawbacks. In addition to several potential side effects, the risk of addiction will always be present. This risk is further exacerbated by tolerance that develops with many patients. In order to overcome these problems, there is a need to identify new compounds that have selective binding toward the three main opioid receptors. Such compounds will also prove useful for designing the next- generation of addiction therapeutics. It has been hypothesized that the hasubanan alkaloids (HB alkaloids) may meet this long-standing need. Unfortunately, the unnatural antipode of these natural products is needed to adequately test this hypothesis. Consequently, an efficient synthetic route to the ent-HB alkaloid core must be developed. The overall objectives of this phased application is to synthesize and evaluate the bioactivity of representative ent-HB alkaloids. Thus, the proposed research is relevant to that part of the NIH's mission that supports research that is fundamental to disease treatment and cure. This PI's objectives will be met by pursuing the following three specific aims. Under the first aim, an efficient synthetic route to the HB alkaloids will be developed and demonstrated by synthesizing two representative family members. This strategy will integrate the formation of the C12-C13 and C14-N bonds with the formation of the B and D rings. Under the second aim, the compounds synthesized in Aim 1 will be evaluated for their ability to function as opioid receptor ligands. This will be accomplished by using standard cell-based assays to determine receptor selectivity and ligand function (agonism or antagonism). Under the third aim, the synthetic route developed in Aim 1 will be adapted in order to synthesis ent-HB alkaloid derivatives with increased potency and selectivity. The identity of the proposed analogs has been guided by the reported X-ray crystal structures of the three different opioid receptors. These analogs have been designed to strengthen interactions with key residues in the conserved region of the binding pocket (to increase potency) and to establish an interaction with a key residue found only in the opioid receptor (to increase selectivity). The expected outcome of this work will be a demonstration that the ent-HB alkaloids do indeed have potent affinity toward the opioid receptors. This will have a positive impact on human health because ent-HB alkaloid analogs can then be used as a new class of probe molecules for identifying new treatments for acute pain. With the proper modifications, these same probe molecules will also prove useful for the development of new treatments for drug addiction.

Public Health Relevance

The studies contained in this application concern the synthesis and evaluation of new natural product-based opioid receptor ligands. Ultimately, this work will serve as a foundation for designing new small molecule treatments for severe pain and new medicines for treating substance abuse.

National Institute of Health (NIH)
National Institute on Drug Abuse (NIDA)
Exploratory/Developmental Grants (R21)
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Special Emphasis Panel (ZRG1)
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Rapaka, Rao
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University of Minnesota Twin Cities
Schools of Arts and Sciences
United States
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