Neutropenia is a dose limiting toxicity of chemotherapy. It results in delay and dose reduction both of which adversely affect outcomes of treatment. Myeloid growth factors are biological agents that stimulate the production go granulocytes and offset the myelosupressive effect of chemotherapy. Two myeloid growth factors are available Granulocytic colony stimulating factor (G-CSF) and granulocytic monocytic colony simulating factor (GM-CSF). This article will discuss G-CSF as it is used more often than GM-CSF. Commertially available G-CSF is made by recombinant DNA technology and may be produced in E. coli (Filgrastim) or chinese hamster ovary cell lines (lenograstim). The half life of filgrastim can be increased by covalently linking it to polyethylene glycol (PEG) and converting it to pegfilgrastim.
Mechanism of Action of G-CSF
G-CSF is a 174 amino acid peptide the gene for which is on chromosome 17. It has a molecular weight of 18kDa. It is produced by monocytes, macrophages, fibroblasts, endothelial cells and keratinocytes in response to inflammatory cytokines and bacterial endotoxin.
G-CSF acts via the G-CSF receptor. G-CSF receptor is a transmembrane receptor that form a homodimer on binding G-CSF. Activation of G-CSF receptor results in activations of JAK/STAT, SRC family of kinases, PI3/AKT and Ras/ERK 1/2. The details of the pathway are not completely understood.
Activations of G-CSF has the following effects that lead to increased production of neutrophils
- Increased Proliferation of Neutrophilic precursors
- Shortened neutrophilic precursor bone marrow transit time
- Functions maturation of neutrophils – increased chemotaxis, phagocytosis and antibody dependent cytotoxicity
G-CSFs Available for Clinical Use
G-CSFs for clinical use is manufactured by recombinant DNA technology. Two molecules are available for clinical use. Filgrastim is produced using E. coli and lenograstim is obtained from Chinese hamster ovarian cells. Lenograstim is glycosylated (4% glycosylation).
Filgrastim on subcutaneous administration filgrastim has a half life of 2.5-5.8 hours. The drug is eliminated by uptake be G-CSF receptors on neutrophils and glomerular filtration. Pegylation, that involves attaching a 20kDa polyethylene glycol (PEG) molecule to the N terminal eliminates renal elimination prolonging the half life to 27-47 hours. The product, pegfilgrastim, is only eliminated by binding to neutrophil G-CSF receptors, patients with low neutrophil counts have a lower clearance. Prevention of glomerular filtration allows administration of pegfilgrastim only once in a chemotherapy cycle.
Indications for G-CSF
The discussion that follows applies to filgrastim and perfilgrastim as these drugs are used more commonly than lenograstim. The general principle apply to lenograstim but readers are advised to refer to information on lenograstim for details of use and adverse effects.
- Primary prevention of febrile neutropenia (FN) in patients with non-Myeloid malignancy on chemotherapy: Patients where on chemotherapy protocols that have a risk of febrile neutropenia equal to or greater than 20% should be administered G-CSF.
- Prevention of recurrence of febrile neutropenia: G-CFS may be used to prevent recurrence of febrile neutropenia in patients who have had an episode of infection in a previous chemotherapy cycle.
- Treatment of patients with febrile neutropenia: Initiating therapy with G-CSF after febrile neutropenia has set in has not been shown to decrease mortality of antibiotic use. It may however be used in patients who are at high risk of mortality.
- Mobilisation of stem cells for stem cell transplant
- Use in patients with myeloid malignancies: There is an apprehension that G-CSF may stimulate leukaemia cells and G-CSF is not used in induction. It may however be used after induction to reduct the duration of neutropenia.
Filgrastim is administered in a dose of 5μg/kg/day subcutaneously, by a short iv infusion or prolonged intravenous infection. therapy should be initiated at least 24 hours after the chemotherapy. The adult dose of pegfilgrastim is 6mg. The paediatric dose depends on the weight of the child. Children less than 10 kg: 0.1 mg/kg, those between 10 to 20 kg be administered 1.5 mg, between 21 to 30 kg be administered 2.5 mg and between 31 to 44 kg administered 4 mg. Children weighing 45kg or more should be administered the adult dose of 6 mg. Pegfilgrastim should not be administered less than 14 days after a cycle of chemotherapy. It should be administered more than 24 hours after a cycle of chemotherapy.
- Bone Pain: Bone pain is the commonest side effect with about 20-30% of the patients suffering the side effect.
- Rare but serous side effects include splenic rupture, acute respiratory distress syndrome, precipitation of sickle cell crisis and capillary leak syndrome
Drugs, prescription and non-prescription, and nutritional supplements are a common cause of eosinophilia across the world. In regions with a low prevalence of parasitic infestations drugs are the leading cause of eosinophilia.
Clinical Spectrum of Drug Induced Eosinophilia
The spectrum of drug induced eosinophilia extends from an asymptomatic eosinophilia discovered on a routine haemogram to a a serious disorder like drug induced drug reaction with eosinophilia and systemic syndromes (DRESS). Eosinophilia associated with specific organ complications includes
- Eosinophilic pulmonary infiltrates associated with the use of sulfadsalazine, nitrofurantoin and non-steroidal anti-inflammatory drugs (NSAID)
- Acute interstitial nephritis with eosinophilia associated with the use of semisynthetic penicillins, cephalosporins, NSAID, sulphonamides, phenytoin, cimetidine and allopurinol
- Eosinophilia-myalgia syndrome (EMS) presents with increased eosinophil counts associated with severe myalgia, neuropathy, skin rash and multi-system complications. The cause of EMS is not known but L-tryptophan has been implemented.
- Drug reaction with eosinophilia and systemic symptoms /Drug induced hypersensitivity syndrome (DRESS/DIHS): The syndrome is a form of delayed drug hypersensitivity the presents with fever lymphadenopathy and end organ damage. The spectrum of end-organ damage includes hepetitis, interstitial nephritis, pneumonitis and carditis. The drugs implicated in DRESS/DIHS include
- Antibiotics: Cephalosporins, doxycycline, fluoroquinolone, linezolid, metronidazole, nitrofurantoin, penicillins, tetracycline
- Sulfomaides: Sulfasalazine trimethoprim-sulfamethoxozole
- Sulfones: Dapsone
- Antiviral: Abacavir, Nevirapine
- Anti-epileptic: Carbamazepine, lamotrigine, phenobarbital, phenytoin, , valproate
- Anti-depressants: Amitriptyline, desimipramine, fluoxetine
- Anti-inflammatory: Diclofenac, ibuprofen, naproxen, piroxicam
- Antihypertensives: ACE inhibitors, β-blockers, hydrochlorthiazide
- Others: Allopurinol, cyclosporine, ranitidine
The incriminating drug should be withdrawn in symptomatic patients. Asymptomatic eosinophilia does not necessitate discontinuation of therapy. If equally effective therapy is available it is preferable to stop therapy. If this is not the case the drug may be continued with careful monitoring for symptoms.
Hydroxyurea was synthesised in Germany in 1860 and was found inhibit granulocyte production. It was only a hundred years after this that its potential as an anticancer drug was realized.
Mechanism of Action of Hydroxyurea
Hydroxyurea enters the cell by passive diffusion. It inhibits of ribonucleotide reductase (RR). RR converts ribonucleotide diphosphates to deoxyribonucleotide diphosphates. Deoxyribonucleotide diphosphates are converted to deoxyribonucleotide triphosphates and incorporated into DNA. Depletion of deoxyribonucleotide triphosphates results in impaired DNA synthesis. RR has two subunits M-1 and M2. The M-2 subunit is the catalytic subunit and contains iron. Hydroxyurea inhibits RR by chelating iron. Hydroxyurea is an S phase specific drug. The cells exposed to hydroxyurea progress normally through the cell cycle, have a normal G1-S transition but accumulate in the S phase because of an inability to synthesise DNA. They then undergo apoptosis by p53 dependent and independent mechanisms. Hydroxyurea may be transformed to nitric oxide. Nitric oxide is also an inhibitor of RR and may be responsible for drugs ability to induce foetal haemoglobin. This is important for treatment of sickle cell anaemia. Resistance to HU develops by elevated cellular activity of RR.
Pharmacokinetics of Hydroxyurea
Oral bioavailability of hydroxyurea is 80-100%. Parenteral formulation has no advantage over oral formulation. The drug is well distributed. It enters breast milk, cerebrospinal fluid and third space collections. The ratio of plasma to CSF levels is 4-9:1 and plasma to ascites levels is 2-7.5:1. The elimination half-life is 3.5-4.5 hours. Renal elimination is the main pathway of elimination. Sixty to eighty per cent of the dose eliminated by kidney unchanged. Patients with creatinine clearance of 10-50ml/hr should receive 50% and those with creatinine clearance of less than 10ml/hr should receive 20% of the planned dose. Hydroxyurea is metabolized but the metabolic pathways are not known.
- Myeloproliferative diseases
- Chronic myeloid leukaemia
- Essential thrombocytosis
- Polycythaemia Vera
- Acute leukaemia to control counts
- Sickle cell anaemia
Myelosuppression is the dose limiting effect of hydroxyurea. The dose of hydroxyurea needs to be titrated to the leucocyte and platelet counts. The acceptable lower limits of these counts will depend on the indication but generally speaking a leukocyte count less than 2.5X109/L or a platelet count less than 100X109/L is an indication for discontinuing therapy. With the abovementioned provisions in mind the dose of hydroxyurea for different indications are as follows:
- Myeloproliferative diseases: The usual dose is 20-30mg/kg/day.
- Acute leukaemia: 50-100mg/kg per day
- Sickle cell anaemia: 15-20mg/kg/day
- HU inhibits formation of deoxynucleotides and enhanced the effect of agents damaging the DNA, as no nucleotides are available for repair. The effects of purine and pyrimidine analogues. When hydroxyurea is combined with any of these agents it should be done as a part of a protocol whose toxicity has been evaluated. This will prevent unacceptable toxicity.
- It has been shown to be synergistic with agents damaging the DNA like cisplatin, alkylating agents and topoisomerase II inhibitors.
- It has been used as a radiosensitizing agent in the treatment of head and neck and cervical cancer. It depletes the deoxynucleotide pool needed for DNA repair after radiation-induced damage.
- Enhanced anti HIV activity of azidothymidine, dideocytidine and dideoxyinosine
- Myelosuppression: The dose limiting toxicity of hydroxyurea is myelosuppression. Hydroxyurea causes rapid fall in leucocyte counts. When used in non-haematological malignancies the fall in leucocyte counts is evident by days 2-5. When used in patients with leukaemia the fall is evident faster, sometimes within a day. This property of hydroxyurea is useful in myeloid leukaemia with very high leucocyte count. Hydroxyurea is the treatment of choice for patients with chronic myeloid leukaemia presenting with very high counts. Though used in acute myeloid leukaemia with hyperleucocytosis, benefit from its use has not been proven in clinical trials.
- Gastrointestinal: Oral ulceration and gastrointestinal tract effects may be seen in some patients. They are particularly common in patients who receive chemoradiation with hydroxyurea.
- Skin: Dermatological changes may be seen with prolonged use. These include
- Skin Pigmentation and rash: Hyperpigmentation, erythema of the face and hands, diffuse maculopapular rash and dry skin. Severe reactions may resemble lichen planus.
- Nail Changes: The nails may show atrophy and formation of multiple pigmented bands.
- Leg Ulcers: Leg ulcerations may be seen in patients with prolonged therapy with hydroxyurea.
- Alopecia: Alopecia may occasionally be seen with the use of hydroxyurea
- Radiation Recall: Erythema or pigmentation of previously radiated skin may be seen in some patients.
- Mutagenicity and Teratogenicity: Hydroxyurea is a proven teratogen and contraindicated in women are pregnant or are planning a pregnancy. Women in the reproductive age group must be advised about contraception. The carcinogenic potential of hydroxyurea is uncertain. In view of the mechanism of action it is prudent not to use hydroxyurea for non-malignant disease.