A CRISPR Dropout Screen Identifies Genetic Vulnerabilities and Therapeutic Targets in Acute Myeloid Leukemia

Pharmacologic studies were performed in man with tritiated 1-β-D-arabinofuranosyl cytosine (ara-C). Most of the plasma and urine radioactivity was in 1-f3-D-arabinofuranosyl uracil (ara-U), and all of the radioactivity could be accounted for as ara-C and ara-U. The plasma disapperance curve of ara-C when plotted on a semilog scale was biphasic with an initial fast phase (mean half-time 12 minutes) and a slower second phase (mean half-time 111 minutes). There was a linear correlation between the dose and plasma concentration of ara-C. The initial plasma ara-U to ara-C ratio was lower at the high doses, suggesting saturation of ara-C deaminase activity. Eighty per cent of the radioactivity was recovered in the urine by 24 hours, 8 per cent as ara-C, and 72 per cent as ara-U. Prior treatment with ara-C had no effect on the pharmacology of ara-C. The plasma ara-C deaminase activity was very low. The total amount in plasma was less than one per cent of that present in the liver. The ara-C deaminase activity of red blood cells (RBC) of normal subjects was 7 per cent that of the liver, and the RBC of solid tumor patients contained significantly higher enzyme activity. Following a loading dose, ara-C was given by constant intravenous infUSion. This resulted in constant plasma ara-C levels which were dose related. At steady state the cerebrospinal fluid (CSF) to plasma ratio of ara-C was 0.4. Intrathecal injection of ara-C resulted in high initial CSF levels and an exponential decrease with a half-time of 2 hours. This very long half-time as compared to the plasma is a function of the very low ara-C deaminase activity in CSF and brain.

Clinical pharmacology of l-β-D-arabinofuranosyl cytosine

Marine natural products, the secondary or nonprimary metabolites produced by organisms that live in the sea, have received increasing attention from chemists and pharmacologists during the last two decades. Interest on the part of chemists has been twofold: natural products chemists have probed marine organisms as sources of new and unusual organic molecules, while synthetic chemists have followed by targeting these novel structures for development of new analogs and new synthetic methodologies and strategies (Albizati et al., 1990). The rationale for investigating the chemistry of marine organisms has changed over the past several decades. Early investigations were largely of a “phytochemical” nature, reporting detailed metabolite profiles similar to those reported for terrestrial plants in previous decades. However, analogous to investigations of terrestrial plants, more recent studies of marine organisms have focused on their potential applications, particularly to the treatment of human disease and control of agricultural pests (Fautin, 1988). Pharmacological evaluations of marine natural products have likewise undergone an evolution over the past two decades: beginning with the early investigations of toxins, followed by studies of cytotoxic and antitumor activity, to the present day, where a myriad of activities based on whole-animal models and receptor-binding assays are being pursued. The intent of this chapter is to look back at the evolution of biomedically oriented natural product studies of marine organisms, to chronicle the key developments, discoveries, and advances in the level of sophistication that have fueled further interest in this field, and finally to look forward at the future biomedical potential of marine natural products.

Biomedical Potential of Marine Natural Products

The antimetabolite cytosine arabinoside (ara-C) represents a prototype of the nucleoside analog class of antineoplastic agents and remains one of the most effective drugs used in the treatment of acute leukemia as well as other hematopoietic malignancies. The ability of ara-C to kill neoplastic cells is regulated at three distinct but interrelated levels. First, the activity of ara-C depends on conversion to its lethal triphosphate derivative, ara-CTP, a process that is influenced by multiple factors, including nucleoside transport, phosphorylation, deamination, and levels of competing metabolites, particularly dCTP. Second, the antiproliferative and lethal effects of ara-C are linked to the ability of ara-CTP to interfere with one or more DNA polymerases as well as the degree to which it is incorporated into elongating DNA strands, leading to DNA fragmentation and chain termination. Finally, the fate of the cell is ultimately determined by whether a threshold level of ara-C-mediated DNA damage is exceeded, thereby inducing apoptosis, or programmed cell death. The latter process is influenced by components of various signal transduction pathways (e.g., PKC) and expression of oncogenes (e.g., bcl-2, c-Jun), perturbations in which may significantly alter ara-C sensitivity. A better understanding of these factors could eventually lead to the development of novel therapeutic strategies capable of overcoming ara-C resistance and improving therapeutic efficacy.

Ara-C: Cellular and Molecular Pharmacology

https://science.sciencemag.org/content/sci/194/4264/491.full.pdf

Drugs from the sea

Summary1-β-D-Arabinofuranosylcytosine hydrochloride has been shown to be active against recently transplanted and established Sarcoma 180, Ehrlich carcinoma and L-1210 leukemia in mice. This nucleoside was inactive at the same dose per kilogram of body weight in tumor-bearing rats.

Antitumor activity of 1-beta-D-arainofuranosylcytosine hydrochloride.

Summary 1-β-d-Arabinofuranosylcytosine hydrochloride (Cytosine arabinoside, CA) was shown to be an effective inhibitor of Sarcoma 180, Ehrlich carcinoma, leukemias L1210 and L1210/C95, lymphoblast L5178Y, and the T-4 lymphoma. CA showed no significant activity against a spectrum of rat tumors or leukemias. Well established mouse tumors and leukemias, as well as recently transplanted neoplasias, were inhibited by the drug. 6-Mercaptopurine- and azaserine-resistant lines of Sarcoma 180 were inhibited by CA, indicating that the sites of metabolic action of the three drugs differ. Combination therapy with CA and 6-MP inhibited the normal and 6-MP-resistant lines of Sarcoma 180. The inhibition appeared to be more than additive. Comparison of the activity of CA with other drugs used in the laboratory and the clinic showed that cytosine arabinoside is one of the most effective agents used in the control of recently transplanted or well established leukemias.

https://cancerres.aacrjournals.org/content/canres/24/7/1285.full.pdf

The effect of 1-beta-d-arabinofuranosylcytosine hydrochloride on murine neoplasms.

The reversal of cytosine arabinoside activity in vivo by deoxycytidine.

Synergism of the antineoplastic activity of cytosine arabinoside by porfiromycin.

Two cell lines of Sarcoma 180 (S-180), CCRF-I and CCRF-II, which do not require added tryptophan for growth in cell culture were studied for their grwoth characteristics in vivo . S-180 ((CCRF-I) regressed rapidly when implanted subcutaneously in pathogen-free mice. The mice whose tumors had regressed were resistant to subsequent challenge with the more virulent S-180 (TUC) and Ehrlich's carcinoma tumors.

Preliminary serologic studies using complement fixation and anaphylaxis tests showed that rabbit antisera to CCRF-I and TUC-S180 cells reacted differently when tested against whole cell antigens. The differences noted in these studies may be quantitative in nature and due to changes in cell surface protein.

Gordon D. Mengel

Papers

Antitumor activity of 1-beta-D-arainofuranosylcytosine hydrochloride.

The effect of 1-beta-d-arabinofuranosylcytosine hydrochloride on murine neoplasms.

The reversal of cytosine arabinoside activity in vivo by deoxycytidine.

Synergism of the antineoplastic activity of cytosine arabinoside by porfiromycin.

Immunological Studies with the Use of Nutritional Variants of Sarcoma 180