Skeletal Pain Research Articles

Phase I Study of ON 01910.Na, a Novel Modulator of the Polo-Like Kinase 1 Pathway, in Adult Patients With Solid Tumors

We conducted a first-in-man (to our knowledge) phase I study to determine the dose-limiting toxicities (DLTs), characterize the pharmacokinetic profile, and document any antitumor activity of ON 01910.Na, a new chemical entity that arrests cancer cells in G(2)/M by modulating mitotic regulatory pathways including polo-like kinase 1 (Plk1). Patients had solid tumors refractory to standard therapy. ON 01910.Na was administered as a 2-hour infusion on days 1, 4, 8, 11, 15, and 18 in 28-day cycles. The starting dose was 80 mg, and an accelerated titration schedule (single-patient cohorts) was used for escalation. Pharmacokinetics were studied on days 1 and 15 of cycle 1. Twenty patients (11 women and nine men; age 46 to 73 years) were enrolled onto the study. Dose levels of 80, 160, 320, 480, 800, 1,280, 2,080, and 3,120 mg were evaluated in single-patient cohorts. A DLT and additional grade 2 toxicities made the 4,370-mg dose (n = 6) not tolerable, and the next lower dose cohort (3,120 mg) was expanded to six assessable patients. Toxicities were skeletal, abdominal, and tumor pain; nausea; urge to defecate; and fatigue. Hematologic toxicity was infrequent and mild. ON 01910.Na pharmacokinetics were characterized by a rapid distribution phase (distribution half-life, 1 hour) and a relatively slow elimination phase (elimination half-life, 27 hours). A refractory ovarian cancer patient had an objective response after four cycles and remained progression free for 24 months. ON 01910.Na showed a distinct but moderate toxicity pattern. The recommended phase II dose of ON 01910.Na with this schedule of administration is 3,120 mg. Single-agent activity was documented in an ovarian cancer patient.

Intérêt de l’imagerie scintigraphique, à Madagascar, dans le diagnostic de l’adénome parathyroïdien ectopique – à propos d’un cas

This study reports the first case of ectopic parathyroid adenoma, diagnosed in the Department of Nuclear Medicine in Antananarivo. This clinical vignette illustrates the interest of the MIBI-Tc-99 m scan in locating this adenoma and its diagnostic confirmation after six years of erratic diagnosis. A whole body bone scintigraphy has also allowed to assess the state of bone metabolism and study outbreaks of fracture. The parathyroid scintigraphy was carried out after intra-venous administration of 666 MBq of MIBI-Tc99m. Dynamic images, static early and late static were acquired with a gamma camera E-Cam Siemens. The whole body bone scan was carried out after administration of 555 MBq of MDP-Tc-99 m. The results evidenced the presence of an para-aortic increased uptake area pointing to a left parathyroid adenoma. The persistence of a late left submaxillary increased uptake area raises, however, a reservation about the existence of a second adenoma. The bone scan displayed global skeletal remodeling, non suggestive of metastases, as it was mentioned with the CT-scan. In a diagnostic tools limited environment, skeletal pain refractory to painkillers, a chronic hypercalcemia associated with an increased parathyroid hormone level, should trigger the scintigarphic exploration in order to avoid bone and renal complications.

Use of Cinacalcet HCl to Achieve the Recommended Targets of Bone Metabolism in a Patient With Therapy-Resistant Renal Hyperparathyroidism

We report on a patient with end-stage renal disease and severe progressive secondary hyperparathyroidism, whose condition failed to respond to conventional pharmacologic or surgical interventions. Although immunotherapy produced a partial response, it failed to decrease serum parathyroid hormone to the levels recommended by the National Kidney Foundation Kidney Disease Outcomes Quality Initiative clinical practice guidelines. Treatment with a new calcimimetic agent, cinacalcet HCl (Mimpara, Amgen, Munich, Germany), resulted in a rapid decline in elevated parathyroid hormone levels, near normalization of other laboratory markers of bone metabolism, improvement in mobility and skeletal pain caused by renal osteodystrophy, and an increase in body weight.

Skeletal features of primary hyperoxaluria type 1, revisited

The purpose of this study was to describe the skeletal manifestations of primary hyperoxaluria type 1 (PH1), the most common of the primary hyperoxalurias. We clinically and radiographically reviewed 12 consecutive patients diagnosed with PH1, aged between 2 and 17 years. All patients had evidence of some type of renal involvement, 4 of whom were at end-stage renal disease (ESRD) and were under dialysis. The main symptom was skeletal pain and was present only in the 4 severely involved patients and appeared during the second year of dialysis. The 2 most severely involved patients had evidence of pathological fractures. Radiological signs were present in patients with or without symptoms. These radiological signs were of two distinct types: those almost specific of oxalosis, such as dense and radiolucent metaphyseal bands and vertebral osteocondensations, which are found mainly in the severely involved individuals, and those less specific, such as signs of renal osteodystrophy, which are also found in less severely involved patients. Interestingly, our study revealed the presence of spondylolysis in 25% of cases. This latter finding is unique and has not previously been reported in the literature. The skeletal manifestations of PH1 include specific and less specific radiological signs, with some patients being asymptomatic, and others presenting with bone pain and pathological fractures, as well as spondylolysis.

Bevacizumab in combination with cisplatin and docetaxel as first line treatment of patients (pts) with advanced or metastatic, non squamous, non-small-cell lung cancer (NSCLC)

19109 Background: Bevacizumab (B) has been shown to improve response rates and survival in pts with advanced non-squamous NSCLC when combined with carboplatin and paclitaxel (FDA & EMEA-approved indication) or with cisplatin and gemcitabine (EMEA-approved). However, there are limited data on the safety and efficacy of B in combination with other widely used chemotherapy doublets for NSCLC. This is a preliminary safety report of a single-arm, open-labeled, phase II trial of B, docetaxel and cisplatin for pts with NSCLC. Methods: In this single-stage phase II trial, patients received docetaxel(75 mg/m2), cisplatin (75 mg/m2), and B (15 mg/kg IV) on day 1 every 3 weeks for up to 6 cycles followed by B 15 mg/kg alone every 3 weeks until disease progression. Eligibility criteria included chemotherapy-naïve stage IIIB or IV incurable NSCLC of non-squamous histology, PS 0–1, no brain metastases, no history of gross hemoptysis, no evidence of tumour invading major blood vessels, stable cardiac condition and no full dose anticoagulation.The primary endpoint was progression-free survival, with secondary endpoints of TTP, RR, OS and safety. Results: 12 pts have been enrolled with a planned sample size of 47 pts. Baseline characteristics were: 2 females, median age 67 years old (62–72), PS 0 (5 pts) or 1 (7 pts), stage IIIB (2 pt) or IV (10 pts). Median number of cycles was 2 (range 0–4). 11 pts are evaluable for toxicity. Hematological toxicities: 2 pts with grade 4 neutropenia (1 febrile neutropenia), 1 pt with grade 4 leucopenia, 1 pt with grade 3 neutropenia. The most common grade 3/4 non-hematological toxicities included fatigue ( N=1, gr 3), diarrhea (N=1, gr 4), xerostomia (N=1, gr 3), dyspnea (N=1, gr 4), skeletal pain (N=1, gr 3), hemoptysis (N=1, gr 5) and flebitis (N=1, gr 3). 4/11 pts are off treatment (1 progression, 1 SAEs, 1 protocol violation, 1 docetaxel allergic reaction). 4 patients were evaluable for tumor response, 2 PR, 1 SD, and 1 PD. Conclusions: preliminary analysis shows that the combination of Bevacizumab, docetaxel and cisplatin is safe, well tolerated and shows a trend of promising activity. Enrollment continues and updated results will be presented. No significant financial relationships to disclose.

Stellenwert der Skelettszintigraphie in der Nachsorge des Mammakarzinoms: Eine prospektive Studie über 750 Fälle

In 1987/88 a prospective study was undertaken of 750 patients with postoperative carcinoma of the breast but no evidence as yet of skeletal metastases. A thorough history regarding pain was taken with pre-set questions and an exact clinical examination conducted to reveal any skeletal pain. Subsequently whole-body skeletal scanning was performed, plus additional X-ray films, determination of tumour markers CA-15-3 and CEA, and further tests as indicated. Clinical and imaging results agreed in 649 patients. In 70 patients the clinical suspicion or questionable finding of metastases was not confirmed by radiology or a scan. Bone metastases would have been missed in only 14 patients (1.9%) without a skeletal scan. Eleven of these 14 patients were in high-risk groups (negative hormone receptor status; axillary lymph node metastases). Skeletal metastases were undiscovered in only three of the 750 patients (0.4%). It is thus sufficient to limit skeletal scans to high-risk groups, as long as a careful history of pain is taken and a thorough clinical examination conducted.

A Fracture Pain Model in the Rat

Because of the relative lack of understanding of the mechanisms that drive skeletal pain, the purpose of this study was to adapt a previously validated closed femur fracture model to quantitatively evaluate skeletal pain in female and male rats. Three-month-old female and male Sprague-Dawley rats were anesthetized, and a stainless steel pin was inserted into the intramedullary space of the left femur. Three weeks later, the rats were reanesthetized, and left femoral diaphyses were fractured using a standardized impactor device. At 1-21 days after fracture, skeletal pain was measured by quantitatively assessing spontaneous guarding, spontaneous flinching, and weight bearing of the fractured hind limb. Females and males showed highly robust pain behaviors that were maximal at day 1 after fracture and returned gradually to normal nonfractured levels at days 14-21 after fracture. The magnitude of fracture pain was not significantly different at most time points between female and male rats. In both females and males, the pain-related behaviors were attenuated by subcutaneous morphine in a dose-dependent manner. This model may help in developing a mechanism-based understanding of the factors that generate and maintain fracture pain in both females and males and in translating these findings into new therapies for treating fracture pain.

Treatment options for non-motor symptoms in late-stage Parkinson's disease

Late-stage Parkinson's disease is characterised by patients dependent on caregivers for their activities of daily living, even under the best levodopa benefit. Non-motor signs that overcome the well-known motor signs of Parkinson's disease dominate late-stage Parkinson's disease and few systematic data exist for the treatment of these signs. The objective of this study was to review the treatment options for Parkinson's disease dementia, psychosis, falls, bone fractures, joint and skeletal deformities, pain, orthostatic hypotension, gastrointestinal abnormalities and urological dysfunction in late-stage Parkinson's disease. The study analysed the available controlled clinical trials for the above medical conditions. When absent, data from case series and the authors' own experience was considered. Few controlled clinical trials specifically addressed late-stage Parkinson's disease as a target population. There is a need for therapeutic data on the symptoms that most afflict late-stage Parkinson's disease patients.

Steroid responsive meningitis-arteritis in dogs

Steroid responsive meningitis-arteritis (SRMA) in the dog is a disease well recognised in small animal practice. A typical presentation would be a juvenile dog (less than two years of age) of medium to large breed (often pure bred) with lethargy, pyrexia, inappetence and some degree of axial skeletal pain. Axial skeletal pain is most commonly evident as reluctance to move, low head carriage, cervical or thoracolumbar pain on examination (Fig. 1) and occasionally an arched back. Clinical signs can wax and wane over a period of months. Once a diagnosis has been made the prognosis for remission is usually excellent.

Targeting Nerve Growth Factor in Pain

Chronic pain presents a huge economic and social burden, with existing treatments largely unable to satisfy medical needs. Recently, studies have shown that nerve growth factor (NGF) is a major mediator of inflammatory and neuropathic pain, providing a new therapeutic target. Although originally discovered as a trophic factor for sympathetic and sensory neurons during development, it now appears that in adults, levels of NGF are elevated in many acute and chronic pain conditions. Furthermore, preclinical animal models of inflammatory and neuropathic pain also show increased NGF levels, while the sequestration of NGF alleviates the associated hyperalgesia. The molecular mechanisms involved are being elucidated. This review briefly examines pain signaling pathways and describes currently available analgesics. It then investigates the approaches taken in targeting NGF-mediated pain. Current options being explored include the development of humanized monoclonal antibodies to NGF or its tyrosine kinase receptor TrkA (also known as neurotrophic tyrosine kinase receptor, type 1 [NTRK1]), and the sequestration of NGF using TrkA domain 5 (TrkAd5), a soluble receptor protein that binds NGF with picomolar affinity. Administration of either antibodies or TrkAd5 has been shown to be effective in a number of preclinical models of pain, including cystitis, osteoarthritis, UV irradiation (sunburn), and skeletal bone pain due to fracture or cancer. Other possible future therapies examined in this review include small-molecule TrkA antagonists, which target either the extracellular NGF binding domain of TrkA or its intracellular tyrosine kinase domain.

Musculoskeletal Manifestations in Pediatric Acute Leukemia

In children, acute leukemia (AL) at presentation can mimic several orthopaedic pathologies, so that a variable delay of the correct diagnosis is often reported. To define more clearly the clinical and radiological musculoskeletal manifestations of leukemia in children, 122 affected children referred from 1984 to 1999 to our Pediatric Onco-Hematologic Clinic were retrospectively reviewed. Average age at diagnosis was 6.6 years (from 7 months to 17 years). Seventy-three (60%) were boys and adolescent boys, 49 (40%) were girls and adolescent girls. One hundred two (83.6%) had acute lymphoblastic leukemia, 20 (16.4%) had acute myeloid leukemia. The mean follow-up was 8.2 years for the 104 survivors and 2.5 years for the 18 nonsurvivors. The chi2 test was used to perform the statistical analyses. At presentation, complaints related to the musculoskeletal system were frequent (38.3%), including pain (34.4%), functional impairment (22.9%), limping (12.3%), swelling (10.6%), and joint effusion (5.7%). At presentation, 40.2% of children had at least 1 radiographic abnormality. In order, they were osteolysis (13.1%), metaphyseal bands (9.8%), osteopenia (9%), osteosclerosis (7.4%), permeative pattern (5.7%), pathological fractures (5.7%), periosteal reactions (4.1%), and mixed lysis-sclerosis lesions (2.5%). Different from previous reports, late radiographic lesions were uncommon (5.7%), probably because of milder newer medication protocols. They included avascular necrosis (3.3%), vertebral collapses (1.6%), and osteolysis (0.8%). Both clinical and radiological changes had various and no uniform localization. Poor correlation was found between symptoms and radiological lesions. Survival rates in children with AL were 95.8% at 1 year, 89.6% at 3 years, 85.8% at 5 years, and 83.4% at 10 and at 13 years. Radiographic abnormalities (P = 0.400), type of leukemia (P = 0.291), sex (P = 0.245), and white blood cell count at presentation (P = 0.877) were not prognostic factors. The presence of multiple bone lesions did not affect the survival rate (P=0.632). As early diagnosis significantly decreases morbidity and mortality of AL, the orthopaedist should suspect AL in any child with unexplained persistent skeletal pain or radiographic alterations. Accurate history, general physical examination, and complete blood cell count tests should address the suspicion, which is confirmed by a peripheral and/or iliac crest bone marrow biopsy.

Effects of a Monoclonal Antibody Raised Against Nerve Growth Factor on Skeletal Pain and Bone Healing After Fracture of the C57BL/6J Mouse Femur

A closed femur fracture pain model was developed in the C57BL/6J mouse. One day after fracture, a monoclonal antibody raised against nerve growth factor (anti-NGF) was delivered intraperitoneally and resulted in a reduction in fracture pain-related behaviors of approximately 50%. Anti-NGF therapy did not interfere with bone healing as assessed by mechanical testing and histomorphometric analysis. Current therapies to treat skeletal fracture pain are limited. This is because of the side effect profile of available analgesics and the scarcity of animal models that can be used to understand the mechanisms that drive this pain. Whereas previous studies have shown that mineralized bone, marrow, and periosteum are innervated by sensory and sympathetic fibers, it is not understood how skeletal pain is generated and maintained even in common conditions such as osteoarthritis, low back pain, or fracture. In this study, we characterized the pain-related behaviors after a closed femur fracture in the C57BL/6J mouse. Additionally, we assessed the effect of a monoclonal antibody that binds to and sequesters nerve growth factor (anti-NGF) on pain-related behaviors and bone healing (mechanical properties and histomorphometric analysis) after fracture. Administration of anti-NGF therapy (10 mg/kg, days 1, 6, and 11 after fracture) resulted in a reduction of fracture pain-related behaviors of approximately 50%. Attenuation of fracture pain was evident as early as 24 h after the initial dosing and remained efficacious throughout the course of fracture pain. Anti-NGF therapy did not modify biomechanical properties of the femur or histomorphometric indices of bone healing. These findings suggest that therapies that target NGF or its cognate receptor(s) may be effective in attenuating nonmalignant fracture pain without interfering with bone healing.

The evidence for the spinal segmental innervation of bone

Dermatomes and myotomes are areas of skin and muscle, respectively, that are innervated by single spinal segmental nerves, and reflect a principle of organization that appears in just about every clinical textbook available today. The evidence for the existence of dermatomes and myotomes has a long and substantial history. A lesser known, but similar principle exists for the skeletal system. The term "sclerotome" was first used in the non-embryological sense by Inman and Saunders ([1944] J. Nerv. Ment. Dis. 99:660-667) to define a region of bone and periosteum that is innervated by a single spinal segment. It is used by clinicians in many healthcare settings to aid in the diagnosis and description of a variety of deep and/or skeletal tissue pathologies and pain syndromes. In this article, the evidence for the existence of the sclerotomes is described in detail. Early clinical studies that define the sclerotomes, evidence from studies of the development of skeletal innervation, and the contributions of anatomical and physiological investigations are explored. It is suggested that there is in fact little direct evidence for the existence of discrete spinal segmental innervation patterns for the skeleton.

Bisphosphonates in Paget’s Disease of Bone

Paget’s disease of bone is a chronic focal bone remodeling disorder with distinctive geographic occurnece. In the US, 1.8% of people over 60 years of age are affected. At present, 70% of patients are asymptomatic with the most common presenting symptoms being bone pain, osteoarthritis, and bone deformity. Paget’s disease is usually easily diagnosed by an elevated serum alkaline phosphatase level, a bone scan and skeletal radiographs of area that show increased activity on the bone scan. There is no cure for this disorder; however, a class of drugs, the bisphosphoates, can control the elevated bone remodeling rates, improve osteolytic skeletal lesions and attendant bone pain. The most effective bisphosphonates are the nitrogen-containing compounds, which can be administered orally or intravednously.

Organization of a unique net-like meshwork of CGRP+ sensory fibers in the mouse periosteum: Implications for the generation and maintenance of bone fracture pain

Although bone fracture frequently results in significant pain and can lead to increased morbidity and mortality, it is still not clearly understood how sensory neurons are organized to detect fracture pain. In the present report we focused on the periosteum, as this thin tissue is highly innervated and tightly adherent to the outer surface of bone. To define the organization and distribution of the sensory and sympathetic fibers in the mouse femoral periosteum, we used whole-mount preparations, transverse sections, immunofluoresence and laser scanning confocal microscopy. While both the outer fibrous layer and the inner more cellular cambium layer of the periosteum receive an extensive innervation by calcitonin gene-related peptide (CGRP) and 200-kDa neurofilament (NF200) positive sensory fibers as well as tyrosine hydroxylase (TH) positive sympathetic fibers, there is a differential organization of sensory vs. sympathetic fibers within the periosteum. In both layers, the great majority of TH+ fibers are closely associated with CD31+ blood vessels and wind around the larger vessels in a corkscrew pattern. In contrast, the majority of CGRP+ and NF200+ sensory fibers in both layers lack a clear association with CD31+ blood vessels and appear to be organized in a dense net-like meshwork to detect mechanical distortion of periosteum and bone. This organization would explain why stabilization/fixation causes a marked attenuation of movement-evoked fracture pain. Understanding the organization, plasticity and molecular characteristics of sensory and sympathetic nerve fibers innervating the skeleton may permit the development of novel mechanism-based therapies for treating non-malignant skeletal pain.

Nerve growth factor sequestering therapy attenuates non-malignant skeletal pain following fracture

Current therapies to treat skeletal fracture pain are extremely limited. Some non-steroidal anti-inflammatory drugs have been shown to inhibit bone healing and opiates induce cognitive dysfunction and respiratory depression which are especially problematic in the elderly suffering from osteoporotic fractures. In the present report, we developed a closed femur fracture pain model in the mouse where skeletal pain behaviors such as flinching and guarding of the fractured limb are reversed by 10mg/kg morphine. Using this model we showed that the administration of a monoclonal antibody against nerve growth factor (anti-NGF) reduced fracture-induced pain-related behaviors by over 50%. Treatment with anti-NGF reduced c-Fos and dynorphin up-regulation in the spinal cord at day 2 post-fracture. However, anti-NGF treatment did not reduce p-ERK and c-Fos expression at 20 and 90min, respectively, following fracture. This suggests NGF is involved in maintenance but not the acute generation of fracture pain. Anti-NGF therapy did not inhibit bone healing as measured by callus formation, bridging of the fracture site or mechanical strength of the bone. As the anti-NGF antibody does not appreciably cross the blood–brain barrier, the present data suggest that the anti-hyperalgesic action of anti-NGF therapy results from blockade of activation and/or sensitization of the CGRP/trkA positive fibers that normally constitute the majority of sensory fibers that innervate the bone. These results demonstrate that NGF plays a significant role in driving fracture pain and that NGF sequestering therapies may be efficacious in attenuating this pain.

Effect of intravenous pamidronate therapy on everyday activities in children with osteogenesis imperfecta

To evaluate the effect of intravenous pamidronate therapy on everyday activities, well-being, skeletal pain and bone density in children with osteogenesis imperfecta (OI). In a prospective observational study, monthly intravenous pamidronate infusions were given to 43 children (aged 4 months-16 years) with different severity and form of OI. Outcome measures included the Pediatric Evaluation of Disability Inventory (PEDI), a score of well-being, registration of days with pain and bone density measured by using dual energy X-ray absorptiometry (DXA). Data were collected before and after the first year of treatment. Self-care and mobility measured by PEDI increased significantly in both the Functional Skill scale and in the Caregiver Assistance scale. The youngest children improved more than the older ones according to the PEDI. The days with skeletal pain were reduced and both well-being and bone density increased. Intravenous treatment with bisphosphonates influenced health status, according to the International Classification of Functioning, Disability and Health (ICF). This group of 43 children experienced beneficial effects on everyday activities, skeletal pain, well-being and bone density.

Two cases of isolated symptomatic myocarditis induced by all-trans retinoic acid (ATRA)

Two cases of isolated symptomatic myocarditis induced by all-trans retinoic acid (ATRA)

Juvenile Paget's Disease: The Second Reported, Oldest Patient Is Homozygous for the TNFRSF11B “Balkan” Mutation (966_969delTGACinsCTT), Which Elevates Circulating Immunoreactive Osteoprotegerin Levels

The oldest person (60 yr) with juvenile Paget's disease is homozygous for the TNFRSF11B mutation 966_969delTGACinsCTT. Elevated circulating levels of immunoreactive OPG and soluble RANKL accompany this genetic defect that truncates the OPG monomer, preventing formation of OPG homodimers. Juvenile Paget's disease (JPD), a rare autosomal recessive disorder, features skeletal pain, fracture, and deformity from extremely rapid bone turnover. Deafness and sometimes retinopathy also occur. Most patients have diminished osteoprotegerin (OPG) inhibition of osteoclastogenesis caused by homozygous loss-of-function defects in TNFRSF11B, the gene that encodes OPG. Circulating immunoreactive OPG (iOPG) is undetectable with complete deletion of TNFRSF11B but normal with a 3-bp in-frame deletion. We summarize the clinical course of a 60-yr-old Greek man who is the second reported, oldest JPD patient, including his response to two decades of bisphosphonate therapy. Mutation analysis involved sequencing all exons and adjacent mRNA splice sites of TNFRSF11B. Over the past 4 yr, we used ELISAs to quantitate his serum iOPG and soluble RANKL (sRANKL) levels. Our patient suffered progressive deafness and became legally blind, although elevated markers of bone turnover have been normal for 6 yr. He carries the same homozygous mutation in TNFRSF11B (966_969delTGACinsCTT) reported in a seemingly unrelated Greek boy and Croatian man who also have relatively mild JPD. This frame-shift deletes 79 carboxyterminal amino acids from the OPG monomer, including a cysteine residue necessary for homodimerization. Nevertheless, serum iOPG and sRANKL levels are persistently elevated. Homozygosity for the TNFRSF11B "Balkan" mutation (966_969delTGACinsCTT) causes JPD in the second reported, oldest patient. Elevated circulating iOPG and sRANKL levels complement evidence that this deletion/insertion omits a cysteine residue at the carboxyterminus needed for OPG homodimerization.

LIFE CYCLE OF HEPATOZOON CANIS (APICOMPLEXA: ADELEORINA: HEPATOZOIDAE) IN THE TICK RHIPICEPHALUS SANGUINEUS AND DOMESTIC DOG (CANIS FAMILIARIS)

The life cycle of the apicomplexan protozoon Hepatozoon canis in its natural hosts Rhipicephalus sanguineus (tick) and Canis familiaris (domestic dog) was studied in an experimental infection. Tick nymphs were fed on a naturally infected dog, or they were infected by percutaneous injection of blood. Dogs were inoculated by ingestion of adult ticks containing mature oocysts. Gamonts were in syzygy 24 hr after percutaneous injection of ticks. Early oocysts were detected 96 hr after nymph repletion, and mature oocysts in adult ticks were infective to dogs 40 days postmolt. Merogony was detected in dog bone marrow from 13 days postinoculation (PI) and included meronts containing 20-30 micromerozoites, and a second type with 2-4 macromerozoites. Monozoic cysts were observed in the spleen in conjunction with merogony. Gamontogony with infection of leukocytes by micromerozoites occurred from 26 days PI, and gamont parasitemia, which completed the life cycle, was detected 28 days PI. The length of the life cycle from nymphal attachment to parasitemia in dogs was 81 days. Increased body temperatures were evident from 16 to 27 days PI and paralleled the time of intensive bone marrow merogony. Skeletal pain and recumbency were manifested in 2 dogs. This study further elucidates the life cycle of H. canis and provides a sequential morphologic description of H. canis merogony, gamontogony, and sporogony.