Synthesis and antitumor activity of the hexacyclic camptothecin derivatives

Topoisomerase Inhibitors and Targeted Delivery in Cancer Therapy

DNA topoisomerases are enzymes that catalyze the alteration of DNA topology with transiently induced DNA strand breakage, essential for DNA replication. Topoisomerases are validated cancer chemotherapy targets. Anticancer agents targeting Topoisomerase I and II have been in clinical use and proven to be highly effective, though with significant side effects. There are tremendous efforts to develop new generation of topoisomerase inhibitors. Targeted delivery of topoisomerase inhibitors is another way to reduce the side effects. Conjugates of topoisomerases inhibitors with antibody, polymer, or small molecule are developed to target these inhibitors to tumor sites.

Semisynthesis, cytotoxic activity, and oral availability of new lipophilic 9-substituted camptothecin derivatives

Despite that 9-substituted camptothecins are promising candidates in cancer therapy, the limited accessibility to this position has reduced the studies of these derivatives to a few standard modifications. We report herein a novel semisynthetic route based on the Tscherniac-Einhorn reaction to synthesize new lipophilic camptothecin derivatives with amidomethyl and imidomethyl substitutions in position 9. Compounds were evaluated for their antiproliferative activity, topoisomerase I inhibition, and oral availability. Preliminary data demonstrated that bulky imidomethyl modification is an appropriate lipophilic substitution for an effective oral administration relative to topotecan. In addition, this general procedure paves the way for obtaining new camptothecin derivatives.

Progranulin regulates neuronal outgrowth independent of sortilin

Background

Progranulin (PGRN), a widely secreted growth factor, is involved in multiple biological functions, and mutations located within the PGRN gene (GRN) are a major cause of frontotemporal lobar degeneration with TDP-43-positive inclusions (FLTD-TDP). In light of recent reports suggesting PGRN functions as a protective neurotrophic factor and that sortilin (SORT1) is a neuronal receptor for PGRN, we used a Sort1-deficient (Sort1^−/−) murine primary hippocampal neuron model to investigate whether PGRN’s neurotrophic effects are dependent on SORT1. We sought to elucidate this relationship to determine what role SORT1, as a regulator of PGRN levels, plays in modulating PGRN’s neurotrophic effects.

Results

As the first group to evaluate the effect of PGRN loss in Grn knockout primary neuronal cultures, we show neurite outgrowth and branching are significantly decreased in Grn^−/− neurons compared to wild-type (WT) neurons. More importantly, we also demonstrate that PGRN overexpression can rescue this phenotype. However, the recovery in outgrowth is not observed following treatment with recombinant PGRN harboring missense mutations p.C139R, p.P248L or p.R432C, indicating that these mutations adversely affect the neurotrophic properties of PGRN. In addition, we also present evidence that cleavage of full-length PGRN into granulin peptides is required for increased neuronal outgrowth, suggesting that the neurotrophic functions of PGRN are contained within certain granulins. To further characterize the mechanism by which PGRN impacts neuronal morphology, we assessed the involvement of SORT1. We demonstrate that PGRN induced-outgrowth occurs in the absence of SORT1 in Sort1^−/− cultures.

Conclusion

We demonstrate that loss of PGRN impairs proper neurite outgrowth and branching, and that exogenous PGRN alleviates this impairment. Furthermore, we determined that exogenous PGRN induces outgrowth independent of SORT1, suggesting another receptor(s) is involved in PGRN induced neuronal outgrowth.

Effect of pH and human serum albumin on the cytotoxicity of a glucuronide prodrug of 9-aminocamptothecin

PURPOSE

9-aminocamptothecin glucuronide (9ACG) is a prodrug of 9-aminocamptothecin (9AC) that displays potent antitumor activity against human tumor xenografts in nude mice. Camptothecins exist in a pH dependent equilibrium between active lactone and inactive carboxy forms that can be altered by binding to human serum albumin (HSA). Here we investigated the influence of pH and HSA on the lactone-carboxy equilibrium, HSA binding, and cytotoxicity of 9ACG.

METHODS

Microfiltration and HPLC were used to measure the influence of pH on lactone to carboxy conversion and HSA binding of 9ACG as compared to other camptothecins. In vitro cytotoxicity of drugs was determined against EJ human bladder carcinoma cells and CL1-5 human lung cancer cells.

RESULTS

The rate of lactone to carboxy conversion was similar for 9ACG and 9AC. Decreasing the pH from 7.6 to 6.0 increased the equilibrium levels of the lactone forms of the drugs from 20 to almost 95% of total drug. HSA moderately diminished the amount of free 9ACG lactone but did not change the ratio of 9ACG lactone to 9ACG carboxy. Consistent with the effect of pH on lactone levels, lowering the pH of EJ human bladder carcinoma cells from 7.6 to 6.8 decreased the IC(50) of 9ACG from 480 to 98 nM and 9AC from 33 to 12 nM. Activation of 9ACG by human beta-glucuronidase anchored on the surface of EJ cells further decreased its IC(50) value to 26 nM. Although HSA significantly decreased the cytotoxicity of 9AC and 9ACG, activation of 9ACG at cancer cells with an antibody-beta-glucuronidase immunoconjugate produced greater cytotoxicity than 9AC.

CONCLUSIONS

Acidification and targeted delivery of beta-glucuronidase can enhance 9ACG cytotoxicity even in the presence of HSA.