Non-Intuitive Medicine: Using Anti-D to Treat ITP

Most people are aware of the consequences of Rhesus incompatibility. It's usually the 2nd pregnancy. The mother is Rhesus negative. The father is Rhesus positive and has the D antigen. The foetus inherits the D antigen from the father. The mother's immune system mounts an attack on the D iso-antigens in the child's blood, resulting in haemolysis, sometimes ending fatally in erythroblastosis foetalis.

Now countenance this. Would you consider infusing a patient known to be Rhesus positive with pre-formed antibodies to the Rhesus(D) antigen, deliberately risking haemolysis?

No, I thought not.

Yet, that is exactly what happens in in subjects with severe Idiopathic Thrombocytopenic Purpura (ITP) who have life threatening haemorrhage. If the patient is D+, he/she is infused with anti-D containing immunoglobulin. IgG antibodies to the D antigen agglutinate the subjects' RBCs. Macrophages, mainly in the spleen, recognise the Fc portion of the anti-D immunoglobulins to bind to the D antigen on RBC and take them up actively. In so doing, the reticulo-endothelial system becomes saturated with millions of antibody-agglutinated RBC and is no longer free to bind to and destroy the platelets.

Medicine in reverse.

Of course, a better known agent for treating ITP is intravenous immunoglobulin (IVIG). It was from observing the small incidence of haemolysis in subjects receiving IVIG that the idea of using anti-D arose. Anti-D has a number of advantages over IVIG. These include a substantially shorter infusion time, markedly smaller infusion volume, longer duration of response demonstrated in HIV-infected patients with ITP, lower cost, and exposure to 1/100th as many donors per dose. Increased platelet count is seen in 70-80% of patients receiving anti-D. The dose is 50-75 ug/kg. It is licensed for this purpose in the USA but not in Europe, where plasmapheresis and romiplostim (thrombopoetin agonist) are used in addition to IVIG for treating major bleeding (intracranial or gastrointestinal) in severe ITP.

Anti-D is not without its dangers with such usage. Approximately 1 in 1000 patients develop severe transfusion reactions, with extravascular haemolysis, and increased risk of DIC and acute renal failure. This has resulted in a Black Box warning being issued by the FDA for such usage. However, in most patients, the degree of haemolysis is mild, with Hb dropping by 0.5-2g within a week, and recovering by 3 weeks.

Understandably, anti-D should not be used in those with pre-existing haemolysis, in those with a positive Coomb's test or those with renal impairment. It is ineffective in those who have had a splenectomy. Some haematologists never use anti-D despite its advantages, and prefer IVIG instead.

IgG anti-D does not bind complement. There is therefore no intravascular haemolysis. Haptoglobin and haemopexin do not fall.

References:

1. Despotovic JM, Lambert MP, Herman JH et al. RhIG for the treatment of immune thrombocytopenia: consensus and controversy. Transfusion 2012;52:1125-6.

Testosterone Replacement Therapy & Risk of Prostate Cancer

Hormone sensitive metastatic prostate cancer is treated with androgen deprivation therapy, typically with gonadotropin releasing hormone analogues, which essentially turn off the pituitary release of luteinizing hormone through sustained stimulation (as opposed to pulsatile release, which is physiological). This results in chemical castration.

The issue assumes relevance when you consider the practice of testosterone replacement therapy (TRT). In recent years, TRT has become popular among primary care physicians, as a sort of panacea for fatigue, lassitude and muscle wasting, particularly in elderly subjects. The FDA recently mandated that preparations for TRT explicitly carry a warning against a higher risk of myocardial infarction and stroke in users. Nevertheless, a subset of subjects with low serum testosterone genuinely benefit from TRT, particularly when associated with diminished libido, or loss of morning or nocturnal erection despite being otherwise healthy, a condition known as adult onset hypogonadism. Such subjects have low total and free serum testosterone and normal LH levels.

When such subjects are elderly, as they often are, it is logical to ask whether such TRT increases the risk of causing hormone sensitive cancers in the future. In fact, TRT can increase the likelihood of developing breast cancer in those who have another risk factor such as BRCA1 or 2 or Cowden's syndrome. However, such cancers are very uncommon. A much more relevant concern is whether TRT stores up a future risk of prostate cancer.

In a disease which, when metastasised, is treated with surgical, or more commonly, chemical castration, it therefore came as a surprise to learn from RCTs that firstly, low testosterone levels were associated with a higher risk of prostate cancer. Secondly, subjects with adult onset hypogonadism had more advanced prostate cancer at diagnosis by stage, grade and volume. Thirdly, TRT was not associated with a higher risk of prostate cancer in any RCT.

Reference

Mayo Clinic Proceedings, Vol. 91, Issue 7, p908–926

Right Colectomy in Mucocoele of the Appendix to Prevent Pseudomyxoma Peritonei

Appendiceal mucocoele is most often caused by benign mucinous cystadenoma, and less commonly by mucinous adenocarcinoma. The commonest malignant tumour of the appendix used to be carcinoid. This is now changing, as mucinous adenocarcinoma becomes more common.

When mucocoele of the appendix is diagnosed, usually incidentally during cross sectional imaging, it mandates appendicectomy. Untreated, appendiceal mucocoeles can rupture, leading to pseudomyxoma peritonei (PMP), an incurable condition.

PMP caused by rupture of mucocoele is also called Disseminated Peritoneal Adenomucinosis or DPAM. Most authorities only use the term PMP when the underlying aetiology is DPAM. However, others use the term PMP to desribe the dissemination of mucin producing cells from carcinomas of the appendix or colon. The distinction is important, because the latter has a materially worse prognosis.

PMP has a striking appearance on imaging. On CT, mucin has the same appearance as water, but in addition, there is widespread calcification, scalloping of the liver and spleen, with a predominantly peripheral distribution of lesions.

PMP is difficult to treat. Repeated cytoreductive surgery (CRT) is required. An approach that is gaining in popularity among surgeons is cytoreductive surgery followed by Heated Intraperitoneal Chemotherapy or HIPEC. The latter involves intraoperative infusion of dialysis fluid containing mitomycin C, heated to 41 degree Celsius to increase penetration of tumour deposits. However, even with heating, the chemotherapeutic agent is unable to penetrate beyond a couple of millimeters.

Pioneering work in this area has been done by Sugarbaker & colleagues. They found that pre-operatively, the prognosis was worse when there was segmental obstruction of the jejunum or proximal ileum or if the peritoneal deposits exceeded 5 mm in size.

When CRT with HIPEC is used, it is standard to follow through with postoperative intra-peritoneal fluorouracil.

Untreated. PMP is fatal as intraperitoneal mucin accumulates inexorably and causes intestinal obstruction. The most well known victim was Audrey Hepburn, who succumbed to this condition.

PMP is more common in women. In the article in JAMA Surgery referenced below, the tumour (mucinous cystadenoma) involved the base of the appendix. To avoid spilling mucin intra-operatively and thus cause PMP, resection margins must be clear. Thus, right colectomy was opted for rather than a simple appendicectomy.

This is an example of a benign tumour behaving like a malignant one.

(Free access is available to most articles in all JAMA group journals through a single registration giving access to the JAMA Network Reader. Thank you, the American Medical Association).

References.

1. http://learningradiology.com/notes/gunotes/pseudomyxomacorrect.htm
2. http://jamanetwork.com/journals/jamasurgery/article-abstract/2601315
3. UpToDate

Enterobacteriaceae and Other Gram Negative Bacilli & Beta-Lactam Resistance

Gram negative bacilli can be resistant to various beta lactams, ranging from penicillins to carbapenems. Such resistance may be constitutive or acquired. Constitutive or in-built resistance is typically due to restricted entry of antibiotics into cells, by "gating" of entry points called porins, which span the outer hydrophobic cell wall in gram negative bacilli. The more hostile an environment the bacterium lives in, the more impermeable will be the porins. Thus, porins in Acintobacter, which lives in liquid media such as irrigation solutions in hospitals, is 100 times less permeable than E.coli, which lives in the relatively friendly and protected environment of the gut with a host of other Enterobacteriaceae. Here, it is worth mentioning that Acintobacter and Pseudomonas, albeit being gram negative bacilli, are not from the same family as Enterobacteria. That is to say, unlike E.coli, Klebsiealla, Citrobacter, Providentia, Serratia, Enterobacter and Proteus species, Pseudomonas and Acintobacter do not live in the gut.

Although not important for beta-lactams, which act in the peri-plasmic space between the inner and outer membranes, antibiotic resistance in gram negative bacteria is also served by an efflux pump, which simply pumps out intracellular antibiotics to the exterior. This mechanism is obviously more important for antibiotics which act inside the cell, such as aminoglycosides, quinolones, tetracyclines and macrolides.

Resistance can also be acquired ecologically, from other successful bacteria, for example in the gut. The most important example of this is the so called "accessory genome" epitomised by plasmids.

Some plasmids are so successful that they can self transmit during bacterial reproduction. They are called constitutive plasmids. Plasmids have their own genetic code called replicons. Plasmids from the same group have similar replicons, and are incompatible with each other. When two similar plasmids are in the same cell, they will only multiply once the cell has divided and the competing plasmids have passed into separate daughter cells.

Constitutive plasmids cause the bacterium in which they reside to become "addicted" to them. Such "plasmid addiction" often involves a toxin-antitoxin system. The toxin tends to be stable while the anti-toxin is unstable and subserved by the plasmid. When the plasmid is lost, so is the anti-toxin, resulting in destruction of the bacterium by the intracellular toxin. In this way, the plasmid ensures that only those bacteria that carry it, survive.


The Ambler Classification for gram-negative resistance to beta-lactams is widely used and comprises four classes of beta-lactam resistance.

Class A- Comprises both broad spectrum beta-lactamases encoded by genes such as TEM and SHV and ESBLs, such as CTX-M-15 (Cefotaxime hydrolysing, first described in Munich). These group of bacteria are sensitive under lab and clinical conditions to inhibition by beta-lactamase inhibitors such as clavulanate, sulbactam, tazobactam and avibactam.

Class A also includes carbapenamases (CPEs) such as KPC (Klebsiella pneumoniae carbapenamase).

Class B- Comprises MBLs or bacteria producing metallo-beta-lactamases, most of which contain zinc as the "metal". Examples include imipenamase (IMP), Verona-integron encoded metallo beta lactamase (VIM), and the New Delhi Metallo-beta-lactamase (NDM). These group of bacteria will hydrolyse almost all beta lactams except aztreonam (a monobactam). They are relatively insensitive to beta lactamase inhibitors.

Class C- These group of bacteria are called Amp-C (or ampicillin hydrolysing). Amp-C bacteria produce cephalosporinases that hydrolyse 1st, 2nd and 3rd generation cephalosporins. Their ability to hydrolyse 2nd gen cephalosporins, called cefamycins, comprising cefoxitin and cefotetam, sets them apart from ESBLs, which cannot hydrolyse cefamycins. This latter property is used in the lab to differentiate Amp-C from ESBL. Amp-C are also insensitive to clavulanate.

Class D- called OXA or oxacillinases. These bacteria are not inhibited by clavulanate either. They hydrolyse 3rd gen cephalosporins but with a differential between cefotaxime and ceftazidime.

There are various ways of screening for beta-lactamases. Paricularly important is to detect ESBL or carbapenamase producing strains, as their spread will lead to widespread bacterial resistance. Isolation methods are often used for such patients.

The two most widely used are the Combined Disc test and the modified Hodge test.

The combined disc test is a variation of the Double Disc Synergy Test (DDST).

Here, two separate 30 ug discs containing the bacterial inoculum are placed 30 mm from each other (centre to centre) in agar. One disc contains cefotaxime, while the other contains amoxycillin-clavulanate. For ESBL producing bacteria (typically strains of E.coli, K.pneumoniae, K.oxytoca or Proteus species), there will be synergy between cefotaxime and clavulanate and the area of bacterial lysis in the cefotaxime disc will be greater where it apposes the clavulanate disc. (Remember, ESBLs as well as carbapenamases belong to Ambler Class A, which is the only class of beta-lactamases significantly inhibited by clavulanate). The appearance of the cefotaxime disc therefore takes on the shape of a keyhole when ESBL or carbapenamase is present.

The problem arises when there are other classes of beta lactamases that hydrolyse 3rd gen cephalosporins, thus masking the presence of ESBL. It's still important to pick up ESBL due to epidemiologic and clinical reasons. When ESBL is being co-produced with significant amounts of Amp-C (Ambler Class C), the classical synergy with clavulanate may not be seen, as Amp-C is not inhibited by clavulanate. This limitation can be overcome by bringing the discs closer than 30 mm, or by adding cloxacillin to the agar, as this inhibits Amp-C.

When ESBL is being co-produced with Ambler Class B, the metallo-beta-lactamases, it is possible to neutralise the latter with EDTA (chelates Zinc), thus bringing out the synergy with the clavulanate disc.

Unfortunately, no tests exist, except PCR, to tell apart the co-existence of ESBL and Ambler Class D- OXA.

Pulmonary Hypertension & Left Main Coronary Compression

Beware of attributing worsening chest pain resembling angina, decreasing exercise tolerance, malignant arrhythmias or new onset heart failure in subjects with known pulmonary hypertension (PH) to the pre-existing condition.

Occasionally, an enlarged pulmonary artery (PA) can compress the ostium of the left main coronary artery (LMCA) and thus lead to worsening angina or angina equivalent such as dyspnoea. Most cases have been described in subjects with longstanding secundum type ASD, where RV volume overload gradually transforms into pressure overload, but the phenomenon has been reported with other causes of PH leading to PA enlargement. The latter exists if the PA diameter exceeds 33 mm or the PA:Aortic diameter exceeds 1.0.

The LMCA arises from the left posterior sinus of Valsalva (the RCA arises from the anterior sinus, which is not paired). The LMCA is uniquely vulnerable to compression by the enlarging PA, particularly when its ostium is higher than usual.

The diagnosis can be made by CT (MDCT) of the heart, or on coronary angiography with the left anterior oblique view.

Unlike atherosclertic LMCA occlusion, where CABG is almost always favoured, the condition responds well to PCI and stenting.

See the latest issue of JAMA Cardiology for a case report.

Biotin, Avidin and False Positive Tests for Grave's Disease

Some thirty years ago, as 2nd year medical students, a couple of us were sat outside our ramshackle hostel in the gathering dusk. Biochemistry exams were in the offing and we were quizzing each other just to make sure that we were all keeping up. Somebody piped up about the importance of not eating raw eggs. The prevailing dogma, which we all agreed upon then, was that biotin in the egg yolk binds to an egg protein called avidin. Cooking the egg broke the bond, which made biotin available to the body, where it subserved carboxylation reactions as a co-enzyme to acetyl co-A carboxylase and pyruvate carboxylase.

As it happens, we were wrong. I succumbed to atavism and purchased the latest edition of Harper's Biochemistry a few years ago. Biotin deficiency is virtually unknown in medicine. You would have to be a habitual eater of raw egg whites, eschewing the yolk, to get even close to being Biotin deficient.

However, elsewhere in the world, some other people took note of the affinity between biotin and avidin and thought of an application that would change immuno-assays for generations to come. It thus came about that biotin is used as a tag. Biotin has a valeric acid side chain that can be modified to make it reactive and conjugate it to almost any protein or nucleic acid. By then adding avidin or an avidin like bacterial molecule called Streptavidin, produced by Streptomyces avidinii to the mixture, you can home in to the protein that has been bound to biotin. Of course, you need to find where the avidin goes. So you label the avidin (or streptavidin) with a "dye" (or a fluorophore, to be more technical), or an enzyme, which will change colour once its substrate is added. And presto, by taking advantage of one of the strongest known non-covalent affinities in nature, every biotin molecule, attached to the protein that you seek to find, will be picked out like a guided missile by the avidin.

The biotin-streptavidin immunossay has become the standard in most labs for procedures such as ELISA, immunohistochemistry etc.

You can do the same thing to find RNA or DNA, known as Northern blotting or Southern blotting. Here, avidin is tagged to a nucleic acid probe that hybridises with the DNA or RNA bound by biotin. However, these blotting procedures have largely been supplanted by PCR.

But that's not the end of the story. Over the recent years, biotin has become quite a popular supplement for people with mitochondrial disorders and in general as a "pick up" as a component of multivitamins. It definitively found its way into mainstream medicine when trials showed that mega-doses of biotin had salutary effects on the functioning of subjects with multiple sclerosis. It is now increasingly prescribed for MS.

Couple of years ago, reports started filtering in of bizarre lab results, particularly TFTs, but also other endocrine tests such as testosterone, FSH and LH and vitamin B12 among others, in subjects who were on biotin supplements. Remarkably the free T3 and T4 would be very high, the TSH suppressed, and levels of TRAb would be high- an exact biochemical mimic of Grave's disease....except that these subjects were completely euthyroid.

It is now appreciated that pharmacologic doses of biotin interferes with the biotin-streptavidin assay. In competitive immunoassays, the substances being measured such as free T3, or free T4, will be falsely high, whereas substances measured in "sandwich immunoassays", such as TSH, PTH and PSA, will be very low.

There have been a number of instances where subjects on biotin supplements have been erroneously treated for Graves Disease, based on such lab results. There are two excellent letters in the latest edition of New England Journal of Medicine that expand on these. They are free to access. One can avoid such misrepresentation of lab results by stopping the biotin supplements 72 hours before running TFTs or other blood tests.

I shared this as it took me back three decades to that long forgotten, dilapidated hostel where we grew up as wide-eyed medical students, completely unaware of how things were about to explode from bench to bedside.

References:

1.Kummer S, Hermsen D, Distelmaier F. Biotin treatment mimicking Graves’ disease. N Engl J Med 2016;375:704-706
2. Trambas CM, Sikaris KA, Lu ZX. A caution regarding high-dose biotin therapy: misdiagnosis of hyperthyroidism in euthyroid patients. Med J Aust 2016;205:192-192

SLE & Complement Deficiency- Where Do We Stand?

The effect of complement deficiency is hierarchical in SLE, with 90% of those with deficiency of C1q, 80% with C4 deficiency and 30% with C2 deficiency developing the disease. While those with C1q deficiency develop classical SLE, with a high incidence of lupus nephritis, deficiency of C4 or C2 lead to unusual clinical and immunological phenotypes. Patients with deficiency of either of these two early complement components present with rash, but no nephritis, have low titre ANA, often in a speckled pattern, do not have antibodies to dsDNA, and have a high prevalence- more than 50%- of anti-Ro antibodies.

C4 and C2 are both in linkage disequilibrium with the MHC antigens. They are positioned on the short arm of chromosome 6 between HLA Class B and DR antigens and are part of HLA Class III molecules along with factor B. Homozygosity for the null allele of C2 is present in 1 in 20,000 Caucasions, and as stated above, around 30% of these subjects develop SLE.

The inheritance of C4 is unusual. Most people have not one, but two alleles of C4 on 6p, designated as C4A and C4B. The two C4 alleles are part of a 4-gene cassette called RCCX which is subject to duplication or deletion, leading to Copy Number Variation, such that individuals may have 1-8 C4 alleles across the two chromosomes. Most Caucasians have 2-4 functioning alleles. Thus, total deficiency of C4 is exceedingly uncommon.

Deletion of C4B has not been associated with lupus, but when both copies of C4A are deleted, there is a higher prevalence of lupus compared with those whose C4A copies are intact. Deletion of C4A is seen as part of extended haplotype HLA B8, SC01, DR3. (S stands for S allele of Factor B, C for C-allele of C2, 0 for null-allele of C4A and 1 is an allele of C4B). Thus, subjects who are homozygous for this extended haplotype are at greater risk of SLE.

The extended haplotype HLA B8, SC01, DR3 is one of the commonest extended haplotypes found in Caucasians, with a haplotype frequency of around 9%. Thus the likelihood of heterozygosity in the Caucasian population using the Hardy Weinberg equilibrium is just under 17%, and the likelihood of homozygosity is 0.8%.

In practice, C3 and C4 levels are measured by most Rheumatologists. C4 levels are normally much lower than C3. A low C4 may be inherited or due to active SLE with complement consumption. An isolated low C4 may thus be difficult to interpret. Total haemolytic complement, or CH50, which is an index of all complement components in the classical pathway and is often measured, will therefore also be low, and will not help in such cases. (CH50 is the reciprocal of the dilution of the patient's plasma which, when added to sheep erythrocytes coated with rabbit anti-sheep antibodies, leads to 50% lysis of the sheep RBC. CH100, which is measured in other labs, is self explanatory). However, complement activation products such as C4d will be raised in complement consumption and low in inherited deficiency without SLE. Complement activation products in plasma are heat labile and technically difficult to measure. Cell bound complement activation products hold a lot of promise.

The HLA B8,DR3 extended haplotype is also associated with Type 1 Diabetes, Coeliac Disease and Adrenal insufficiency as part of polyglandular autoimmune syndrome type II.

Interestingly, just as C4A deficiency is part of an extended haplotype, so is deficiency of the product encoded by another member of HLA Class III, 21-beta hydroxylase. CYP21B is often deleted alongwith C4B as part of two other extended haplotypes. Homozygosity for these haplotypes result in Congenital Adrenal Hyperplasia.

C2 deficiency also occurs as part of an extended haplotype- HLA B18, DR2. Around 1% of Caucasians are heterozygotes. C2 is the only complement component in the classical pathway where heterozygosity is associated with reduced C50. With the others, C50 is only low with a homozygous state.