A phase I clinical trial of prolonged infusion of hydroxyurea in combination with hyperfractionated, accelerated, external radiation therapy in patients with advanced squamous cell cancer of the head and neck

Clinical pharmacology and clinical trials of ribonucleotide reductase inhibitors: is it a viable cancer therapy?

PURPOSE

Ribonucleotide reductase (RR) enzymes (RR1 and RR2) play an important role in the reduction of ribonucleotides to deoxyribonucleotides which is involved in DNA replication and repair. Augmented RR activity has been ascribed to uncontrolled cell growth and tumorigenic transformation.

METHODS

This review mainly focuses on several biological and chemical RR inhibitors (e.g., siRNA, GTI-2040, GTI-2501, triapine, gemcitabine, and clofarabine) that have been evaluated in clinical trials with promising anticancer activity from 1960's till 2016. A summary on whether their monotherapy or combination is still effective for further use is discussed.

RESULTS

Among the RR2 inhibitors evaluated, GTI-2040, siRNA, gallium nitrate and didox were more efficacious as a monotherapy, whereas triapine was found to be more efficacious as combination agent. Hydroxyurea is currently used more in combination therapy, even though it is efficacious as a monotherapy. Gallium nitrate showed mixed results in combination therapy, while the combination activity of didox is yet to be evaluated. RR1 inhibitors that have long been used in chemotherapy such as gemcitabine, cladribine, fludarabine and clofarabine are currently used mostly as a combination therapy, but are equally efficacious as a monotherapy, except tezacitabine which did not progress beyond phase I trials.

CONCLUSIONS

Based on the results of clinical trials, we conclude that RR inhibitors are viable treatment options, either as a monotherapy or as a combination in cancer chemotherapy. With the recent advances made in cancer biology, further development of RR inhibitors with improved efficacy and reduced toxicity is possible for treatment of variety of cancers.

Hydroxyurea: a key player in cancer chemotherapy

Hydroxyurea (HU) is a simple organic compound currently used as a cancer chemotherapeutic agent. It acts specifically on the S-phase of the cell cycle by inhibiting the enzyme ribonucleoside diphosphate reductase, thereby hindering the reductive conversion of ribonucleotides to deoxyribonucleotides and thus limiting de novo DNA synthesis. HU is employed in hemotological settings as a first-line treatment of myeloproliferative disorders, such as polycythemia vera, essential thrombocythemia and primary myelofibrosis, apart from having a vital role in combination therapy for management of malignant melanoma, head and neck cancers and brain tumors. It offers an advantage that the patient may take this drug on an ambulatory basis with minimum clinical toxicity, while some of its limitations include gastrointestinal disturbance and bone marrow depression. This review will summarize and present the overall effects of HU and its combination therapy as an anticancer agent.

Ribonucleotide reductase inhibitors: a new look at an old target for radiosensitization

Ribonucleotide reductase (RR), the rate limiting enzyme in the synthesis and repair of DNA, has been studied as a target for inhibition in the treatment of cancer for many years. While some researchers have focused on RR inhibitors as chemotherapeutic agents, particularly in hematologic malignancies, some of the most promising data has been generated in the field of radiosensitization. Early pre-clinical studies demonstrated that the addition of the first of these drugs, hydroxyurea, to ionizing radiation (IR) produced a synergistic effect in vitro, leading to a large number of clinical studies in the 1970–1980s. These studies, mainly in cervical cancer, initially produced a great deal of interest, leading to the incorporation of hydroxyurea in the treatment protocols of many institutions. However, over time, the conclusions from these studies have been called into question and hydroxyurea has been replaced in the standard of care of cervical cancer. Over the last 10 years, a number of well-done pre-clinical studies have greatly advanced our understanding of RR as a target. Those advances include the elucidation of the role of p53R2 and our understanding of the temporal relationship between the delivery of IR and the response of RR. At the same time, new inhibitors with increased potency and improved binding characteristics have been discovered, and pre-clinical and early clinical data look promising. Here we present a comprehensive review of the pre-clinical and clinical data in the field to date and provide some discussion of future areas of research.

Phase II clinical trial of parenteral hydroxyurea and hyperfractionated, accelerated external beam radiation therapy in patients with advanced squamous cell carcinoma of the head and neck: Toxicity and efficacy with continuous ribonucleoside reductase inhibition

BACKGROUND

Almost all concurrent chemoradiation regimens for head and neck are platinum based; however, cisplatin is associated with severe renal, oto-, and neurotoxicity. Hydroxyurea (HU) has been associated with fewer irreversible toxicities. We obtained HU in parenteral form to be administered continually during the radiation treatment. Intravenous HU promised better pharmacokinetics and cell cycle blockade.

METHODS

Participants had biopsy-proven, untreated squamous cell carcinoma of the oral cavity, oropharynx (stage IV) and hypopharynx (stages II-IV). Radiation therapy consisted initially of 74.4 Gy administered in twice daily 1.2-Gy fractions. After 20 patients, the radiation dose was reduced to 60.0 Gy, and another 16 patients were enrolled.

RESULTS

Patients received HU by Continuous Ambulatory Drug Delivery (CADD) pump on a daily x5 schedule during radiation therapy. Because of persistent long-term dysphagia, after 20 patients, the dose of external beam radiation therapy was reduced from 74 to 60 Gy, and the duration of concurrent HU was correspondingly reduced. The new regimen was much better tolerated. The median survival for the group as a whole was 30 months. Within this small study, there were no significant differences in survival, regional control, or local control between the 2 groups.

CONCLUSIONS

Lower doses of concurrent parenteral HU and hyper-fractionated radiation therapy are tolerable and promising.

Oligopeptide inhibition of class I ribonucleotide reductases

Class I ribonucleotide reductases (RRs), which are well-recognized targets for cancer chemotherapeutic and antiviral agents, are composed of two different subunits, R1 and R2, and are inhibited by oligopeptides corresponding to the C-terminus of R2, which compete with R2 for binding to R1. These peptides specifically inhibit the RRs from which they are derived, and closely homologous RRs, but do not inhibit less homologous RRs. Here we review results obtained for oligopeptide inhibition of RRs from several sources, including related x-ray, NMR, and modeling results. The most extensive studies have been performed on herpes simplex virus-RR (HSV-RR) and mammalian-RR (mRR). A common model fits the data obtained for both enzymes, in which the C-terminal residue of the oligopeptide (Leu for HSV-RR, Phe for mRR) binds with high specificity to a narrow and deep hydrophobic subsite, and two or more hydrophobic groups at the N-terminal portion of the peptide bind to a broad and shallow second hydrophobic subsite. The studies have led to the development of highly potent and specific inhibitors of HSV-RR and promising inhibitors of mRR, and indicate possible directions for the development of inhibitors of bacterial and fungal RRs.

Phase I clinical trial of parenteral hydroxyurea in combination with pelvic and para-aortic external radiation and brachytherapy for patients with advanced squamous cell cancer of the uterine cervix

PURPOSE

Oral hydroxyurea (HU) is a potent radiation sensitizer, but in vitro studies have suggested that prolonged exposure to HU by way of continuous parenteral infusion would enhance clinical efficacy. The objective of this study was to determine the maximal tolerated dose and identify the toxicities of continuous infusion HU in combination with pelvic and para-aortic external beam radiotherapy (RT) and intrauterine brachytherapy in patients with locally advanced carcinoma of the uterine cervix.

METHODS

This Phase I study of concomitant RT was designed with an escalating dose schedule of HU administered by continuous infusion. HU was administered parenterally as a continuous infusion, 5 d/wk, during the first 21 days of external radiation, during the final 5 days of external beam RT, followed by another 5-day infusion schedule bracketing the single fraction of brachytherapy. The maximal tolerated dose was defined as the highest dose level at which 3 of 3 or 5 of 6 patients could be treated without dose-limiting toxicity.

RESULTS

At dose level 1 (0.25 mg/m(2)/min), 0 of 4 patients experienced Grade 4 toxicities and 2 patients experienced Grade 3 hematologic toxicities that were not considered dose-limiting. One of the first 4 patients at level 2 (0.375 mg/m(2)/min) had Grade 3 diarrhea, but the 3 subsequent patients tolerated the dose. At level 3 (0.5 mg/m(2)/min), 4 of 5 patients failed to complete therapy without a >7-day interruption in HU.

CONCLUSIONS

The maximal tolerated dose of parenteral HU was 0.375 mg/m(2)/min when administered with concomitant RT. The most common toxicities were hematologic. A new trial, incorporating concurrent cisplatin, HU, and RT is planned.

Stage 4 squamous cell carcinoma of the tongue in a child: complete response to chemoradiotherapy

This report describes a complete response to a chemoradiotherapy regimen in a child with an advanced and unresectable squamous cell carcinoma of the tongue. An 8-year-old girl had stage 4 squamous cell carcinoma of the tongue (T4N2M0), causing severe trismus and dysphagia. She received hyperfractionated external beam radiotherapy (total 74.4 Gy) and concomitant intravenous infusion of hydroxyurea (0.313 mg/m2 per min) for 43 days. Grade 3 mucositis and myelosuppression were the main toxicities. There was marked symptomatic improvement, and the patient achieved a complete response. She is disease-free 24 months after treatment, and all the acute symptoms have resolved. The regimen was well tolerated with acceptable toxicity and led to a complete objective response. This regimen needs further evaluation to confirm its efficacy and to ascertain its long-term effects in children.

Sequencing cyclic peptide inhibitors of mammalian ribonucleotide reductase by electrospray ionization mass spectrometry

Mammalian ribonucleotide reductase (mRR) is a potential target for cancer intervention. A series of lactam-bridged cyclic peptide inhibitors (1-9) of mRR have been synthesized and tested in previous work. These inhibitors consist of cyclic and linear regions, causing their mass spectral characterization to be a challenge. We determined the fragmentation mechanism of cyclic peptides 1-9 using an ion-trap mass spectrometer equipped with an ESI source. Low-energy collision-induced dissociation of sodiated cyclic peptides containing linear branches follows a general pathway. Fragmentation of the linear peptide region produced mainly a and b ions. The ring peptide region was more stable and ring opening required higher collision energy, mainly occurring at the amide bond adjacent to the lactam bridge. The sodium ion, which bound to the carbonyl oxygen of the lactam bridge, acted as a fixed charge site and directed a charge-remote, sequence-specific fragmentation of the ring-opened peptide. Amino acid residues were cleaved sequentially from the C-terminus to the N-terminus. Our findings have established a new way to sequence cyclic peptides containing a lactam bridge based on charge-remote fragmentation. This methodology will permit unambiguous identification of high-affinity ligands within cyclic peptide libraries.

Synthesis and biological activity of a bivalent nucleotide inhibitor of ribonucleotide reductase

A novel nucleotide inhibitor (ADP-S-HBES-S-dGTP) of mouse ribonucleotide reductase was designed to span the active site and the allosteric specificity site of the enzyme. The inhibitor contains ADP and dGTP moieties which are linked by 1,6-hexane-(bis-ethylenesulfone), and has a Ki value of 12 microM.

Synthesis and biological activity of cyclic peptide inhibitors of ribonucleotide reductase

[formula: see text] A series of lactam-bridged peptide inhibitors (2-6) of mammalian ribonucleotide reductase (mRR) has been designed and synthesized on the basis of the heptapeptide N-AcFTLDADF (1), corresponding to the C-terminus of the R2 subunit of mRR. Inhibition studies revealed a direct relation between ring size and activity, with peptide 5 being 2.5 times more potent than peptide 1.