PF-06952229, a selective TGF-ß-R1 inhibitor: preclinical development and a first-in-human, phase I, dose-escalation study in advanced solid tumors.

BACKGROUND: PF-06952229 is a selective small-molecule inhibitor of transforming growth factor-ß (TGF-ß) receptor 1. We evaluated its antitumor activity in preclinical studies and its safety, tolerability, pharmacokinetics, and pharmacodynamics in a phase I study (NCT03685591). PATIENTS AND METHODS: In vitro and in vivo preclinical studies were conducted. Patients (aged ≥18 years) received PF-06952229 monotherapy [20-500 mg, oral b.i.d., 7 days on/7 days off, 28-day cycles, Part 1A (P1A)] for advanced/metastatic solid tumors and combination therapy [250/375 mg with enzalutamide, Part 1B (P1B)] for metastatic castration-resistant prostate cancer (mCRPC). Primary endpoints were dose-limiting toxicity (DLT), adverse events (AEs), and laboratory abnormalities. Efficacy, pharmacokinetic parameters, and biomarker modulation were assessed. RESULTS: PF-06952229 showed activity in preclinical murine tumor models including pSMAD2 modulation in tumors. The study (NCT03685591) enrolled 49 patients (P1A, n = 42; P1B, n = 7). DLTs were reported in 3/35 (8.6%) P1A patients receiving PF-06952229 375 mg (anemia, intracranial tumor hemorrhage, and anemia and hypertension, all grade 3, n = 1 each). The most frequent grade 3 treatment-related AEs (TRAEs) were alanine aminotransferase increased and anemia (9.5% each). There were no grade 4-5 TRAEs. Plasma PF-06952229 exposures were dose proportional between 80 and 375 mg. Pharmacodynamic studies confirmed target modulation of pSMAD2/3 (peripheral monocytes). One P1A patient with prostate cancer receiving PF-06952229 375 mg monotherapy achieved confirmed partial response (31-month duration of response). A total of 8 patients (P1A, n = 6; P1B, n = 2) achieved stable disease. CONCLUSIONS: Antitumor activity of PF-06952229 was observed in preclinical studies. PF-06952229 was generally well tolerated with manageable toxicity; a small group of patients achieved durable responses and/or disease stabilization.

Effect of rifampin on the pharmacokinetics, safety and tolerability of navitoclax (ABT-263), a dual inhibitor of Bcl-2 and Bcl-XL , in patients with cancer.

WHAT IS KNOWN AND OBJECTIVE: Navitoclax, a first-in-class small molecule Bcl-2 family inhibitor, is metabolized in vitro by the hepatic microsomal cytochrome P450 (CYP) enzymes CYP3A4. Drugs that affect CYP3A4 may therefore have an impact on the pharmacological profile of navitoclax. This study evaluated the effects of co-administration of a potent CYP3A4 inducer rifampin on the pharmacokinetic and safety profiles of navitoclax. METHODS: This open-label, fixed-sequence, 2-period study was performed in twelve subjects with non-haematologic or haematologic malignancy that was relapsed or refractory to standard therapy. A 7-day washout period separated the two treatment periods. On Study Day 1 and Day 8, subjects received a single 250 mg oral dose of navitoclax. Rifampin 600 mg was administrated once daily (QD) on Study Day 4 through Day 10. Blood samples for navitoclax assay were collected prior to dosing (0 h) and at a series of time points through 72 h after dosing on Study Day 1 and Day 8. RESULTS AND DISCUSSION: Co-administration of a single 250 mg dose of navitoclax with 600 mg QD doses of rifampin had a negligible effect on the maximum plasma concentration (Cmax ) of navitoclax [ratio of geometric least square means: 0·84 (90% CI: 0·61-1·16)] but moderately decreased the area under the plasma concentration-time curve (AUC) of navitoclax [ratio of geometric least square means: 0·59 (90% CI: 0·44-0·80)]. Rifampin did not affect the half-life of navitoclax. Co-administration of rifampin did not appear to significantly change the safety profile of navitoclax in the limited number of patients evaluated in this study. WHAT IS NEW AND CONCLUSION: Co-administration navitoclax with rifampin moderately decreased navitoclax AUC, which could be partly due to the induction effect of rifampin on CYP3A4. Further assessment on the mechanism of drug interaction is warranted.

A phase I study evaluating the effect of CDHP as a component of S-1 on the pharmacokinetics of 5-fluorouracil.

UNLABELLED: The purpose of this study was to investigate the effect of gimeracil (CDHP), a reversible dihydropyrimidine dehydrogenase (DPD) inhibitor, on the pharmacokinetics of 5-fluorouracil (5-FU) and other related metabolites by comparing the pharmacokinetic (PK) profile of S-1 (tegafur [FT] + CDHP + oteracil potassium [Oxo]) to that of FT alone. PATIENTS AND METHODS: Patients with advanced solid tumors received single oral doses of S-1 (50 mg) and FT (800 mg) on days 1 and 8 in a randomized crossover fashion. Plasma samples were collected on days 1, 2, 3, 8, 9 and 10. Single-dose PK parameters were determined for FT, 5-FU and α-fluoro-ß-alanine (FBAL). Following the single-dose crossover period, patients entered an extension phase and received treatment with S-1 b.i.d. for 14 days followed by a 7-day rest, repeated every 3 weeks. RESULTS: A total of 12 patients were enrolled; median age was 59 years and mean body surface area was 1.94 m(2). Following S-1 administration, 5-FU exposure was significantly greater (approximately 3-fold) compared to FT alone (p ≤ 0.0007 for AUC0-inf, AUC0-last, and C(max) of 5-FU) despite the 16-fold higher dose of FT administered alone compared to S-1, while plasma concentrations of FT and FBAL were significantly lower with S-1 (p < 0.0001 for all comparisons). Following both single- and multiple-dose administration of S-1, the average maximum DPD inhibition was observed at 4 h post-dose. The extent of inhibition was similar following single and multiple dosing. Following single- and multiple-dose administration of S-1, plasma concentrations of uracil returned to baseline levels within approximately 48 h of dosing, indicating reversibility of DPD inhibition by CDHP. CONCLUSION: Despite the differences in the FT dose administered, exposure to 5-FU was significantly greater following S-1 administration compared to FT administration. Conversely, exposure to FT and FBAL were significantly less following S-1 administration compared to FT administration. Thus, the DPD inhibitory action of CDHP contributes to a decrease in 5-FU catabolism and to significantly higher blood levels of 5-FU compared to FT alone.

A phase I study to determine the safety, pharmacokinetics and pharmacodynamics of a dual VEGFR and FGFR inhibitor, brivanib, in patients with advanced or metastatic solid tumors.

BACKGROUND: This study was designed to determine the safety, pharmacokinetics (PK) and pharmacodynamics (PD) of brivanib in patients with advanced/metastatic solid tumors. PATIENTS AND METHODS: Ninety patients enrolled in this two-part, phase I open-label study of oral brivanib alaninate. The primary objectives of this study were (in part A) dose-limiting toxicity, maximum tolerated dose (MTD) and the lowest biologically active dose level and (in part B) the optimal dose/dose range. The secondary objectives of this study were preliminary evidence of antitumor activity, PK and PD. RESULTS: Across part A (open-label dose escalation and MTD) and part B (open-label dose optimization), 68 patients received brivanib alaninate. Brivanib demonstrated a manageable toxicity profile at doses of 180-800 mg. Most toxic effects were mild. Systemic exposure of the active moiety brivanib increased linearly ≤1000 mg/day. The MTD was 800 mg/day. Forty-four patients were treated at the MTD: 20 with 800 mg continuously, 11 with 800 mg intermittently and 13 with 400 mg b.i.d. doses. Partial responses were confirmed in two patients receiving brivanib ≥600 mg. Dynamic contrast-enhanced magnetic resonance imaging demonstrated statistically significant decreases in parameters reflecting tumor vascularity and permeability after multiple doses in the 800-mg continuous q.d. and 400-mg b.i.d. dose cohorts. CONCLUSION: In patients with advanced/metastatic cancer, brivanib demonstrates promising antiangiogenic and antitumor activity and manageable toxicity at doses ≤800 mg orally q.d., the recommended phase II study dose.

Low-dose trimetrexate glucuronate and protracted 5-fluorouracil infusion in previously untreated patients with advanced pancreatic cancer.

BACKGROUND: 5-Fluorouracil (5-FU)-based regimens have not been shown to prolong survival or provide clinical benefit in patients with advanced pancreatic cancer. The purpose of this study was to determine the tolerability of protracted venous infusion (PVI) of 5-FU, modulated by a low dose of the synthetic antifolate trimetrexate, in patients with advanced pancreatic cancer. PATIENTS AND METHODS: Twenty-three chemotherapy-naïve patients were evaluated. Patients were enrolled in four consecutive cohorts in which the weekly dose of trimetrexate was escalated in 10 mg/m2 increments, from 20 to 50 mg/m2. PVI 5-FU was administered at a fixed dose of 225 mg/m2/day. Treatment was administered for 6 successive weeks, every 8 weeks. RESULTS: Twenty-two patients were assessable. The maximum tolerated dose of trimetrexate was 40 mg/m2. The most common grade 3 and 4 toxicity was diarrhea. There were no treatment-related deaths. Preliminary analysis of activity revealed a response rate of 9%, with 41% of the patients having stable disease for a median duration of 3.8 months. The median survival for the entire group was 6.9 months (range 1-29 months). A clinical benefit response was experienced by 27.2% of patients. CONCLUSIONS: Low-dose trimetrexate can be safely administered in combination with PVI 5-FU. This treatment is well tolerated and is associated with palliative activity in advanced pancreatic cancer.

Zoledronic acid versus pamidronate in the treatment of skeletal metastases in patients with breast cancer or osteolytic lesions of multiple myeloma: a phase III, double-blind, comparative trial.

PURPOSE: Zoledronic acid, a new and more potent bisphosphonate, was compared with pamidronate, the current standard treatment for patients with osteolytic or mixed bone metastases/lesions. PATIENTS AND METHODS: A total of 1,648 patients with either Durie-Salmon stage III multiple myeloma or advanced breast cancer and at least one bone lesion were randomly assigned to treatment with either 4 or 8 mg of zoledronic acid via 15-minute intravenous infusion or 90 mg of pamidronate via 2-hour intravenous infusion every 3 to 4 weeks for 12 months. The primary efficacy endpoint was the proportion of patients experiencing at least one skeletal-related event over 13 months. RESULTS: The proportion of patients with at least one skeletal-related event was similar in all treatment groups. Median time to the first skeletal-related eventwas approximately 1 year in each treatment group. The skeletal morbidity rate was slightly lower in patients treated with zoledronic acid than in those treated with pamidronate, and zoledronic acid (4 mg) significantly decreased the incidence and event rate for radiation therapy to bone, both overall and in breast cancer patients receiving hormonal therapy. Pain scores decreased in all treatment groups in the presence of stable or decreased analgesic use. Zoledronic acid (4 mg) and pamidronate were equally well tolerated; the most common adverse events were bone pain, nausea, fatigue, and fever and < 5% of serious adverse events were related to the study drug. The incidence of renal impairment among patients treated with 4 mg of zoledronic acid via 15-minute infusion was similar to that among patients treated with pamidronate. CONCLUSIONS: Zoledronic acid (4 mg) via 15-minute intravenous infusion was as effective and well tolerated as 90 mg of pamidronate in the treatment of osteolytic and mixed bone metastases/lesions in patients with advanced breast cancer or multiple myeloma. (Can-

A phase I dose-ranging trial of monthly infusions of zoledronic acid for the treatment of osteolytic bone metastases.

Bisphosphonates are potent inhibitors of bone resorption and provide a therapeutic benefit for patients with bone metastases. Zoledronic acid is a highly potent, nitrogen-containing bisphosphonate. In the present trial, we assessed the safety and tolerability of increasing doses of zoledronic acid and its effects on urinary markers of bone resorption in cancer patients with bone metastases. Fifty-nine cancer patients with bone metastases were enrolled sequentially into one of 8 treatment groups in the core protocol. Each patient received a 5-min i.v. infusion of 0.1, 0.2, 0.4, 0.8, 1.5, 2, 4, or 8 mg zoledronic acid monthly for 3 months. Patients were monitored for clinical findings, adverse events, electrocardiograms, markers of bone resorption, as well as routine hematology, blood chemistries, and urinalysis. Thirty patients who demonstrated a radiographic response to treatment or stable disease in the core protocol were enrolled in a humanitarian extension protocol and continued to receive monthly infusions. Zoledronic acid was well tolerated at all dose levels. Adverse events reported by >10% of patients included skeletal pain, nausea, fatigue, upper respiratory tract infection, constipation, headache, diarrhea, and fever. Three patients in the core protocol and one patient in the extension protocol experienced grade 3 skeletal pain, "flu-like" symptoms, or hypophosphatemia, which were possibly related to treatment; all recovered completely. Adverse events were reported with similar frequency across all of the dosage groups. Zoledronic acid resulted in sustained, dose-dependent decreases in urinary markers of bone resorption. Zoledronic acid was safe and well tolerated and demonstrated potent inhibition of bone resorption.

Zoledronic acid reduces skeletal-related events in patients with osteolytic metastases.

BACKGROUND: This study evaluated the dose-response relation for zoledronic acid, a new generation high potency bisphosphonate, given as a 5-minute infusion in patients with malignant osteolytic disease. METHODS: Two-hundred eighty patients with osteolytic lesions due to metastatic breast carcinoma or multiple myeloma were randomized to double-blind treatment with either 0.4, 2.0, or 4.0 mg of zoledronic acid or 90 mg pamidronate. The primary efficacy endpoint was the proportion of patients receiving radiation to bone. Other skeletal-related events, bone mineral density (BMD), bone markers, Eastern Cooperative Oncology Group performance status, pain and analgesic scores, and safety also were evaluated. RESULTS: Zoledronic acid at doses of 2.0 and 4.0 mg and pamidronate at a dose of 90 mg each significantly reduced the need for radiation therapy to bone (P < 0.05) in contrast with 0.4 mg zoledronic acid, which did not. Skeletal-related events of any kind, pathologic fractures, and hypercalcemia also occurred less frequently in patients treated with 2.0 or 4.0 mg zoledronic acid or pamidronate than with 0.4 mg zoledronic acid. Increases in lumbar spine BMD (6.2-9.6%) and decreases in the bone resorption marker N-telopeptide (range, -37.1 to -60.8%) were observed for all treatment groups. Skeletal pain, fatigue, nausea, vomiting, and headache were the most commonly reported adverse events. Adverse events were similar in nature and frequency with zoledronic acid and pamidronate. CONCLUSIONS: A 5-minute infusion of 2.0-4.0 mg zoledronic acid was at least as effective as a 2-hour 90-mg pamidronate infusion in treatment of osteolytic metastases. A 0.4-mg dose of zoledronic acid was significantly less effective. Both zoledronic acid and pamidronate were well tolerated.

Angiogenesis inhibition in solid tumors.

Angiogenesis plays a central role in a variety of physiologic and pathologic disease states. Because the growth and metastasis of malignant neoplasms require the presence of an adequate blood supply, pharmacologic inhibition of tumor-induced angiogenesis represents a promising target for antineoplastic therapy. A number of approaches to such inhibition are therefore under active investigation. Vascular endothelial growth factor (VEGF) and its receptor VEGFR-2 are among the best characterized of the various key elements in benign and neoplastic angiogenesis. In 1997, clinical trials were initiated to evaluate an anti-VEGF monoclonal antibody and a VEGFR-2 antagonist as therapy for patients with different types of solid tumors and hematologic neoplasms. Dose selection for these cytostatic agents requires translation from preclinical models, as these agents are likely to require chronic dosing at an optimal biological dose, rather than a maximally tolerated dose. For example, SU5416, a novel small-molecule inhibitor of VEGFR-2, administered at 145 mg/m2 intravenously twice weekly, is well tolerated and achieves the concentration levels required to inhibit endothelial cell proliferation in preclinical models. Because the mechanism of action of angiogenesis inhibitors is complementary to that of classic cytotoxic chemotherapy, preclinical models and subsequent clinical trials frequently explore combinations of these agents with cytotoxic chemotherapy, hoping to achieve additive or synergistic antitumor activity. It is hoped that the combination of angiogenesis inhibitors with cytotoxic chemotherapeutic agents will significantly improve survival and quality of life for cancer patients. As a result of favorable results from Phase 1 and 2 studies, randomized, multicenter clinical investigations of angiogenesis inhibitors are ongoing.

Irinotecan plus fluorouracil and leucovorin for metastatic colorectal cancer. Irinotecan Study Group.

BACKGROUND: The combination of fluorouracil and leucovorin has until recently been standard therapy for metastatic colorectal cancer. Irinotecan prolongs survival in patients with colorectal cancer that is refractory to treatment with fluorouracil and leucovorin. In a multicenter trial, we compared a combination of irinotecan, fluorouracil and leucovorin with bolus doses of fluorouracil and leucovorin as first-line therapy for metastatic colorectal cancer. A third group of patients received irinotecan alone. METHODS: Patients were randomly assigned to receive irinotecan (125 mg per square meter of body-surface area intravenously), fluorouracil (500 mg per square meter as an intravenous bolus), and leucovorin (20 mg per square meter as an intravenous bolus) weekly for four weeks every six weeks; fluorouracil (425 mg per square meter as an intravenous bolus) and leucovorin (20 mg per square meter as an intravenous bolus) daily for five consecutive days every four weeks; or irinotecan alone (125 mg per square meter intravenously) weekly for four weeks every six weeks. End points included progression-free survival and overall survival. RESULTS: Of 683 patients, 231 were assigned to receive irinotecan, fluorouracil, and leucovorin; 226 to receive fluorouracil and leucovorin; and 226 to receive irinotecan alone. In an intention-to-treat analysis, as compared with treatment with fluorouracil and leucovorin, treatment with irinotecan, fluorouracil, and leucovorin resulted in significantly longer progression-free survival (median, 7.0 vs. 4.3 months; P=0.004), a higher rate of confirmed response (39 percent vs. 21 percent, P<0.001), and longer overall survival (median, 14.8 vs. 12.6 months; P=0.04). Results for irinotecan alone were similar to those for fluorouracil and leucovorin. Grade 3 (severe) diarrhea was more common during treatment with irinotecan, fluorouracil, and leucovorin than during treatment with fluorouracil and leucovorin, but the incidence of grade 4 (life-threatening) diarrhea was similar in the two groups (<8 percent). Grade 3 or 4 mucositis, grade 4 neutropenia, and neutropenic fever were less frequent during treatment with irinotecan, fluorouracil, and leucovorin. Adding irinotecan to the regimen of fluorouracil and leucovorin did not compromise the quality of life. CONCLUSIONS: Weekly treatment with irinotecan plus fluorouracil and leucovorin is superior to a widely used regimen of fluorouracil and leucovorin for metastatic colorectal cancer in terms of progression-free survival and overall survival.

Consensus statement on the live organ donor.

OBJECTIVE: To recommend practice guidelines for transplant physicians, primary care providers, health care planners, and all those who are concerned about the well-being of the live organ donor. PARTICIPANTS: An executive group representing the National Kidney Foundation, and the American Societies of Transplantation, Transplant Surgeons, and Nephrology formed a steering committee of 12 members to evaluate current practices of living donor transplantation of the kidney, pancreas, liver, intestine, and lung. The steering committee subsequently assembled more than 100 representatives of the transplant community (physicians, nurses, ethicists, psychologists, lawyers, scientists, social workers, transplant recipients, and living donors) at a national conference held June 1-2, 2000, in Kansas City, Mo. CONSENSUS PROCESS: Attendees participated in 7 assigned work groups. Three were organ specific (lung, liver, and kidney) and 4 were focused on social and ethical concerns (informed consent, donor source, psychosocial issues, and live organ donor registry). Work groups' deliberations were structured by a series of questions developed by the steering committee. Each work group presented its deliberations to an open plenary session of all attendees. This information was stored and shaped into a statement circulated electronically to all attendees for their comments, and finally approved by the steering committee for publication. The term consensus is not meant to convey universal agreement of the participants. The statement identifies issues of controversy; however, the wording of the entire statement is a consensus by approval of all attendees. CONCLUSION: The person who gives consent to be a live organ donor should be competent, willing to donate, free from coercion, medically and psychosocially suitable, fully informed of the risks and benefits as a donor, and fully informed of the risks, benefits, and alternative treatment available to the recipient. The benefits to both donor and recipient must outweigh the risks associated with the donation and transplantation of the living donor organ.

Irinotecan in lymphoma, leukemia, and breast, pancreatic, ovarian, and small-cell lung cancers.

Irincotecan (CPT-11 [Camptosar] has a board range of antitumor activity. Extensive preclinical and early clinical work has demonstrated its activity against many tumor types--head and neck, esophagus, stomach, pancreas, liver, colon/rectum, kidney, lymph nodes, ovary, uterine, cervix, sarcoma, melanoma, acute and chronic leukemia, mesothelioma, and cancers of unknown primary site. Most of the phase II and III trials have focused on colorectal and other gastrointestinal, non-small-cell lung, and cervical cancers (discussed elsewhere in this monograph). This article presents preliminary results of studies exploring the use of irinotecan in lymphoma, leukemia, and breast, pancreatic, ovarian, and small-cell lung cancers. In all of these studies, the number of patients enrolled is small, drug doses and schedules differ (often within the same case series), and little information is available on response duration and overall survival. Nevertheless, irinotecan has shown reproducible if at times modest activity in almost all of the diseases in which it has been studied. Future research should be directed at conducting well-designed clinical trials of irinotecan alone and in combination with other agents.

Adapting a statewide patient database for comparative analysis and quality improvement.

BACKGROUND: In 1991 the Michigan Health and Hospital Association established the Michigan Patient Outcome Measures Program to support quality improvement activities among member hospitals. Data submitted by each hospital are based on the Uniform Hospital Discharge Data Set (UHDDS). REPORTING THE MEASURES: Each participating hospital is provided with tabulations for each of the 18 indicators, along with comparative data for each of five hospital peer groups. CASE STUDY 1: FACILITATING DEVELOPMENT OF A CLINICAL PATHWAY: Review of data on stroke patients indicated that patients discharged to home represented the single largest group of patients and that their length of stay was lower than for other groups of stroke patients. Hospital A used a clinical pathway to achieve reduction in length of stay of almost a day for this group. CASE STUDY 2: IMPROVING OBSTETRICS AND GYNECOLOGIC SERVICES THROUGH EDUCATION: The Department of Obstetrics and Gynecological Surgery at Hospital B achieved improvement in birth trauma, hysterectomy, cesarean section (C-section) births, and vaginal births after C-section. Program participants are considering adopting a birth trauma indicator. CASE STUDY 3: ESTABLISHING A HOSPITALWIDE QI PROGRAM: The Newborn Team at Hospital C initiated several QI initiatives aimed at reducing the newborn mortality rate, including guidelines for earlier identification of high-risk obstetric cases and for earlier intervention by the perinatalogist. CONCLUSIONS: Data that are not risk or severity adjusted have value in assessing hospital procedures and systems and can be used to educate and effect change in practice patterns.

X-ray diffraction studies of 14-filament models of deoxygenated sickle cell hemoglobin fibers. II. Models based on the deoxygenated sickle hemoglobin crystal structure.

The calculated transforms of a number of crystal-based models of the deoxygenated sickle cell hemoglobin fiber have been compared with X-ray diffraction data of 15 A (1 A = 0.1 nm) resolution. The fiber models consist of 14 single strands of sickle cell hemoglobin (HbS) molecules, which associate into seven protofilaments arranged similarly to those present in the crystal structure. Six of the protofilaments are arranged in three crystallographic until cells extending in the c-axis direction with the seventh protofilament positioned so as to provide an elliptical cross-section when the assemblage is viewed down the fiber axis. Models were generated by systematically and independently translating each of the model's three subcells in steps of 3.5 A along the fiber axis. The seventh protofilament was kept fixed as a point of reference. Each translation of a subcell corresponded to a different fiber model whose transform was then compared with observed data. In all, over 46,000 transforms were computed; of these, three models with minimal residuals were identified. The free energy of packing for all crystal-based models was evaluated to find configurations of protofilaments possessing minimal free energies. The results of the calculations support the subcell configurations of two of the three models with minimal residuals.

Erabutoxin b. Initial protein refinement and sequence analysis at 0.140-nm resolution.

The crystal structure of the protein postsynaptic neurotoxin, erabutoxin b, has been refined at 0.140-nm resolution (R = 0.22) by restrained least-squares and interactive computer graphics. The study has established complete structural identity of the two sea-snake venom toxins, erabutoxin b and neurotoxin b, isolated from Laticauda semifasciata snakes taken in different Pacific Ocean waters. Two chemical-sequence inversion errors in erabutoxin b have been discovered during refinement, corrected and subsequently confirmed in both erabutoxin b and erabutoxin a by chemical analysis. The correct sequences are His6-Gln7, hitherto unsuspected, and Ser18-Pro19. The sequence correction His6-Gln7 explains the anomalous results of 1H NMR solution studies and those of early chemical modification experiments, which were in conflict with the previously published three-dimensional structure of erabutoxin b. On refinement, the five-stranded beta sheet described earlier is now shown to be discontinuous, split into a two-stranded beta loop and a three-stranded beta sheet. Unique features of the Pro44-Gly49 peripheral segment have now been identified. 51 water molecule positions have been located.